Current measurement

ABSTRACT

The present invention relates to current measurement apparatus. The current measurement apparatus comprises first and second measurement devices with each of the first and second measurement devices being operative to measure current in a respective one of a live conductor and a neutral conductor substantially simultaneously. The current measurement apparatus is operative to make plural different determinations in dependence on the substantially simultaneous current measurements.

CLAIM OF PRIORITY

This Application is a continuation of and claims priority to U.S. patentapplication Ser. No. 13/684,213, filed Nov. 22, 2012, which claimspriority to GB Application Serial No. 1120295.9, filed Nov. 24, 2011, aswell as U.S. Provisional Application Ser. No. 61/563,462, filed Nov. 23,2011, which are hereby incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

The present invention relates to current measurement apparatusconfigured in particular but not exclusively for measurement of currentflowing in mains electrical circuits and a current measurement methodusing such current measurement apparatus.

BACKGROUND TO THE INVENTION

Accurate measurement of current is required for different applicationsin the electricity consumption and generation fields. For exampleaccurate current measurement is required for metering of electricityusage or generation, for over current protection and for sub-metering,e.g. in a building in which there are distinct electricity consumers forwhom metering is required on an individual basis.

A current shunt provides one approach to measuring the high values ofcurrent encountered in such applications. In use a shunt of knownresistance is provided in series with a load and the voltage developedacross the shunt by the load drawn current is measured. The currentpassing through the shunt is then determined on the basis of Ohm's Lawin view of the measured voltage and the known resistance of the shunt.Another approach to measuring high values of current involves the use ofa current transformer wound on a core which is disposed around aconductor carrying current to be measured. The Hall current probe andthe Rogowski coil provide further approaches to the measurement of highcurrent. Each of these known approaches offers its advantages anddisadvantages with one approach to current measurement being chosen inpreference to the other approaches in dependence on requirements, e.g.with regards to accuracy, operating environment, space constraints, costand the like.

The current shunt is capable of measuring both AC and DC and providesfor linearity of measurement. Furthermore the current shunt is capableof providing absolute accuracy of measurement and temperature stabilitywhen properly calibrated and fabricated from a material having a verylow temperature coefficient of resistance, such as manganin alloy.Certain applications, such as metering of electricity consumption andgeneration, require measurement to high accuracy over extended periodsof time. For example in North America the ANSI C12.20 standard specifiesan accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% forClass 0.2 consumption meters. Standards applicable in Europe andelsewhere, such as IEC 62053, specify similar accuracy requirements.Initial calibration to high accuracy is therefore normally required. Thecurrent shunt is, however, invasive and provides no isolation. Thecurrent transformer on the other hand provides for isolation and is lessinvasive but is capable of measuring AC only. In addition the currenttransformer is liable to non-linearity and phase error problems.

In contrast with the current transformer the Hall current probe iscapable of measuring both AC and DC. In an open loop configuration theHall current probe is, however, liable to non-linearity and temperaturedrift. When in a closed loop configuration the Hall current probeprovides an improvement with regards to non-linearity and temperaturedrift although the weight and size of the configuration increasessignificantly where higher currents are measured. Turning to theRogowski coil, this approach is entirely non-invasive because the coilis wound around a conductor which is to be the subject of measurement.The Rogowski coil offers the further advantage over the currenttransformer of being less liable to saturation because it lacks the ironcore of the current transformer. However and as with the currenttransformer the Rogowski coil is capable of measuring alternatingcurrent only.

Ground fault conditions can present a risk of electric shock inelectrical systems. Ground fault electric shock conditions can arisewhere there is insufficient grounding within an electrical system. Forexample the casing of electrical equipment may be improperly groundedsuch that when a person touches the casing he presents a lower impedancepath to ground should the casing become live.

Ground fault electric shock conditions can also arise in electricalsystems which meet accepted grounding practice. For example the TTgrounding approach involves providing a ground at the utility pole and aground directly to earth at each item of electrical apparatus. The TTgrounding approach has been widely used in Europe mainly on account ofthe saving in wiring that the approach affords. Under certaincircumstances the TT approach can, however, present problems. Forexample if a lightning surge on the power distribution lines produces asurge current of 1000 Amps which runs to earth at the utility pole, avoltage rise of 25,000 Volts is seen at the grounding electrode at theutility pole assuming the resistance between the grounding electrode andground to be 25 Ohms. A resistance of 25 Ohms from the groundingelectrode to earth meets NEC requirements. Where a first surface onelectrical apparatus is connected to the utility pole ground and asecond surface on the electrical apparatus is connected to a separatelocal ground direct to earth the 25,000 Volt signal appears across thefirst and second surfaces.

Ground fault electric shock conditions can arise even in an electricalsystem that is grounded according an approach, such as TN-C, which incontrast to the TT approach affords risk reduction in the face oflightning strikes and like fault conditions. More specifically and isalmost universally appreciated a ground fault electric shock conditionwill arise when a person becomes the only path to ground for currentflow by, for example, inserting a metal object into an electricalsocket. No amount of grounding precautions will prevent electric shockin such circumstances.

Ground fault detectors are operative to determine if there is leakage ofelectrical current from an electrical circuit. Such leakage arises whenthere is a ground fault condition such as according to one of theexamples given above. A ground fault detector may therefore provide ameans to reduce the risk of electric shock. The ground fault detectoroperates on the basis that outwardly flowing current, e.g. in one ormore live wires, must return, e.g. through a neutral wire, unless thereis a current leakage path. It therefore follows that the sum of thecurrents flowing in conductors to and from an electrical load should bezero unless there is a leak. The differential current transformer is aknown form of sensor which is operative to determine the sum of currentsflowing in conductors to and from an electrical load. The differentialcurrent transformer comprises a core, which extends around the multipleconductors to be measured that form the primary and a multi-turnsecondary winding, which is wound radially around the core. When the sumof the currents in the conductors passing through the core is zero nocurrent signal is induced in the secondary winding. When the sum of thecurrents in the conductors passing through the core is more or less thanzero a proportional current signal is induced in the secondary winding.The differential current transformer therefore provides a measure of theleakage current. A circuit breaker may then be operated in dependence onthe secondary winding current exceeding a threshold value for a periodof time, which corresponds to a maximum level of safe fault current. Theresponse time of a leakage current detector can be in the range of 5 mS,50 mS or 500 mS depending on the level of fault current. A circuitcomprising a current sum sensor and a circuit breaker is termed a GroundFault Circuit Interrupter (GFCI) in the US and a Residual CurrentCircuit Breaker (RCCB) or a Residual Current Device (RCD) amongst otherterms in Europe. Sometimes the RCD term is used with respect to a devicewhich is operative to detect leakage current but which lacks a circuitbreaker.

An arc fault is another form of circuit condition that is liable tocause damage and be prejudicial to safety. An arc fault can generatehigh temperatures and thereby ignite combustible material. There are twoforms of arc fault: the series arc fault; and the parallel arc fault.The series arc fault occurs across a discontinuity in a live or neutralconductor. Such a discontinuity is caused by, for example, a brokenconductor, a loose terminal or a poor electrical connection at a wirenut. The current level in a series arc fault is limited by the impedanceof the load. The parallel arc fault involves arcing between twoconductors, such as between a live conductor and a neutral or groundedconductor, and typically arises when conductor insulation is damaged ordeteriorates over time or through usage. The current level in a parallelarc fault is limited by the current available from the supply as limitedby the impedance of the conductors carrying the fault current. Parallelarc faults therefore often involve higher levels of peak current thanseries arc faults. Furthermore the time constant that determines thelength of time that an arc event is present is relatively short comparedwith other fault events. Typically the time constant is of the order of10 nS, 100 nS, 1 uS or 10 uS depending on the line and load conditions.Therefore the peak current of the arc event may be present forinsufficient time to trigger other fault detectors, such as over currentor ground fault detectors. In addition arc fault determination ofteninvolves the analysis of multiple arc events for their periodicity andfrequency.

The Arc Fault Circuit Interrupter (AFCI) is operative in the samefashion as the GFCI to open one or more ungrounded conductors when anunsafe circuit condition is detected. An arc fault is an intermittentcondition which is characterised by a high peak current value but a lowRoot Mean Square (RMS) current value, which is generally below thenormal operating threshold of a GFCI. In a first form an AFCIconsequently comprises a current sensor, which is operative to measurethe load current in one of the conductors, a waveform analysis circuit,which is operative on the output from the current sensor to discriminatebetween waveforms that are characteristic of normal circuit transients,such as transients caused by operation of wall switches, and waveformsthat are characteristic of risk presenting arcs, and a circuit breakerthat is operative in dependence on detection of an arc. This form ofAFCI is capable of detecting and acting upon series and parallel arcfaults. In a second form the AFCI comprises a differential currenttransformer disposed around the conductors to be monitored instead ofthe current sensor of the first form. A parallel arc fault from aconductor to ground produces a current on one of the conductors only,which is readily detected by the waveform analysis circuit. On the otherhand a parallel arcing condition between the conductors produces equaland opposite currents in the conductors as in the GFCI as describedabove. However there is a phase difference between the current waveformspresent in the two conductors. The differential transformer combinedwith a high pass filter allows the monitoring of any high frequencytransient without need to handle the dynamic range on the lowerfrequency normal waveform. The differential transformer rejects anycommon signal such as the mains load current while passing anydifference as may be caused by the time delay difference between liveand neutral that will typically occur in a series arc fault, to therebyeffectively act as a high pass filter and improve the dynamic rangerequirement to extract an arc event. The current transformer issometimes combined with an extra high pass or band pass filter tofurther select the characteristics of interest for only arc faultdetection. The waveform analysis circuit of the AFCI is thereforeoperative at a sufficiently high frequency to identify and act upon thecurrent waveform present in at least one of the conductors to therebydetect the arc fault.

The most familiar application of electricity measurement is inelectricity consumption metering for invoicing purposes. With thedevelopment of local electricity generation capabilities metering ofgenerated electricity for invoicing purposes is becoming more widelyused. Beyond invoicing, electricity metering sees application in demandmonitoring which is of importance to the electricity generator anddistributor for determining usage patterns and trends. Electricitymetering is also seeing increased use in the smart grid as a means todetermine the behaviour and actions of suppliers and consumers connectedto the grid. As mentioned above certain applications of electricitymetering require measurement to high accuracy over extended periods oftime with the ANSI C12.20 standard in North America specifying anaccuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% for Class0.2 consumption meters. Standards applicable in Europe and elsewherespecify similar accuracy requirements.

Digital electricity meters have been used for some years. Such digitalelectricity meters typically comprise a potential divider formeasurement of voltage. There are different approaches to currentmeasurement depending on circumstances and requirements. Normally acurrent sensor is provided on the live conductor only. In some cases,however, there is a current sensor on each of live and neutral for thepurpose of crude tamper detection. Although such tamper detectarrangements sometimes take account of the measurements on live andneutral they do not do so to any degree of accuracy or provide for faultdetection. One approach involves the use of a shunt resistor in the liveconductor and a current transformer on the neutral conductor. Anotherapproach involves the use of a current transformer on each of the liveand neutral conductors. A further approach involves the use of a shuntresistor on each of the live and neutral conductors with the electricitymeter being configured to maintain isolation between the live andneutral conductors despite the galvanic connection to both live andneutral conductors.

Sub-metering provides for billing of individual consumers where theelectricity utility is unable or unwilling to measure the consumption ofsuch individual consumers. Typical users of sub-metering includeapartment complexes, commercial buildings and mobile home parks.Individual metering of electricity consumption has the advantage ofcreating awareness of energy conservation on the part of the consumer.Alternatively sub-metering can take place at the point of load, i.e. atthe electrical apparatus. Sub-metering at the point of load can providean indication of improper operation of the electrical apparatus, e.g. asreflected by an unusual increase in consumption. Furthermoresub-metering at the point of load provides the consumer with insight asto the extent of consumption of the electrical apparatus in absolute andcomparative terms.

Fault detection, such as by the GFCI and the AFCI, has seen increaseduse over the years as a means to improve upon personal safety and toreduce the incidence of damage to property through fires. The design andoperation of fault detectors is subject to standards created by variousbodies. For example the National Electrical Manufacturers Association(NEMA), which represents the interests of electro-industry productmanufacturers in the US, publishes standards relating to fault detectorsprimarily for the US market. The Underwriters Laboratories (UL) alsopublishes standards for electrical safety equipment. A further exampleis the International Electrotechnical Commission (IEC) which has beenthe primary organisation for creating standards, which althoughinternational in scope are biased towards European practices.Legislation and regulations in certain jurisdictions has been a primarymotivator for increased use of fault detectors. For example Germany hasrequired the use of Residual Current Devices (RCDs) on sockets up to 20Amps from June 2007, Norway has required the use of RCDs in all newhomes since 2002 and all new sockets since 2006 and the UK has requiredRCDs in all new installations since 2008. For ground fault devices thereis an IEC specification, namely IEC 61008 and a UL specification, namelyUL 943. AFCIs have been of greater interest in the US and Canada in parton account of the prevalence of wooden and hence fire damage pronebuildings. In a pan European context, the MID (Measuring InstrumentsDirective) took effect on 30 Oct. 2006 with the aim of creating a singlemarket for measuring instruments across the European Union. The objectsof the MID are to guarantee a high level of safety and reliability forcertified measuring instruments and provide for protection against datacorruption in such measuring instruments whilst providing for freecirculation of measuring instruments within the European Union. Annexesto the MID define how measuring instruments can be certified ascompliant. Notified bodies are authorised to carry out testing ofmeasuring instruments, with certificates issued by a notified body beingaccepted throughout the European Union. The MID supersedes nationalmeasures such as the OFGEM approval process in the UK. As furtherexamples of national measures, the 1999 version of the NationalElectrical Code (NFPA 70) in the United States (US) and the 2002 versionof the Canadian Electrical Code (CSA Standard C22.1) each require AFCIsin all circuits feeding outlets in bedrooms of dwellings. AFCIs aresubject to a UL specification, namely UL1699. A more recent example isthe 2008 National Electrical Code requirement for installation ofcombination-type AFCIs in all 15 and 20 Ampere residential circuits withthe exception of laundries, kitchens, bathrooms, garages and unfurnishedbasements.

The design and operation of fault detectors is less than straightforwardcompared, for example, to the design and operation of over-currentdetectors. More specifically different forms of fault detector, such asthe GFCI and the AFCI, involve different electrical designs. Indeedrequirements may differ within a particular class of fault detector.More specifically a parallel arc fault typically manifests as anintermittent current in excess of 75 Amps whereas a series arc faultmanifests as an intermittent current in excess of 5 Amps. Furthermorethe maximum level of peak current depends on the time constant and theform of electrical circuit in which the device is used. Faultcharacteristics may be difficult to distinguish from the currentconsumption characteristics of equipment normally attached to the sameelectrical network. For example the initial current drawn by a motor mayappear like an arc fault and this may lead to false tripping. Anotherconsideration is the differing requirements from jurisdiction tojurisdiction. For example Class A GFCIs have a minimum must trip valueof between 4 mA and 6 mA in the US whereas the RCD, which is theequivalent device in Europe, has a trip value of 30 mA. Furthermore therequired time to trip often depends on the level of fault current with ahigher level of fault current requiring a shorter time to trip. Improperoperation, such as on account of false triggering, over sensitivity orunder sensitivity, provides for further complication.

In addition requirements differ from electrical installation toelectrical installation. For example one installation may require GFCIand over-current protection whereas another installation may requireGFCI, AFCI and over-current protection along with a current measuringcapability. Such differing requirements are met by installing pluraldevices. Where multiple functionality is required in the deployment ofswitchgear in a building multiple different devices are connected inseries. This is likely to present an issue of cost and size or involvelimiting capabilities by sharing components between or amongst pluralsystems. For example an RCD or sub-meter may be shared amongst severalcircuit breakers.

The present invention has been devised in the light of the inventors'appreciation of the above mentioned problems. It is therefore an objectfor the present invention to provide improved current measurementapparatus configured to measure current in a live conductor and aneutral conductor. It is a further object for the present invention toprovide an improved method of measuring current comprising measuringcurrent in a live conductor and a neutral conductor.

STATEMENT OF INVENTION

In the light of the inventors' above mentioned appreciation andaccording to a first aspect of the present invention there is providedcurrent measurement apparatus comprising first and second measurementdevices, each of the first and second measurement devices beingoperative to measure current in a respective one of a live conductor anda neutral conductor substantially simultaneously, the currentmeasurement apparatus being operative to make plural differentdeterminations in dependence on the substantially simultaneous currentmeasurements.

In use the first and second measurement devices measure the absolutecurrent in the live and neutral conductors substantially simultaneouslyand the current measurement apparatus makes plural differentdeterminations on the basis of the simultaneous current measurements.This approach provides a basis for deriving further measurements, suchas a difference between the currents in the live and neutral conductors,performing computations in dependence on the current measurements andeffecting control of the current measurement apparatus in asubstantially instantaneous or at least a near real time fashion.

A prior art approach is to make use of plural apparatus dedicated to itsrespective function, e.g. first apparatus dedicated to currentmeasurement for power determination purposes, second apparatus dedicatedto ground fault detection and third apparatus dedicated to series and/orparallel arc fault detection. The prior art approach may further involvemaking use of a control unit which is operative to perform computationsand effect control operations on the basis of signals generated by thededicated apparatus. Such a prior art approach normally does not lenditself to substantially instantaneous or near real time operation, whichmay be advantageous, for example, in providing for accurate measurementon a continuous basis, such as for demand monitoring or billingpurposes, or for providing for rapid response to a fault condition, suchas by operating a circuit breaker to prevent damage to property orinjury to personnel. The present invention may also provide a basis forimplementing apparatus of smaller footprint than apparatus according tothe prior art approach. For example the present invention may lenditself to implementation in a form that may be more readily accommodatedin distribution boxes and consumer units where space is limited.Furthermore the present invention may be more cost effective than theplural apparatus approach of the prior art. Furthermore the presentinvention may provide for development of new and more intelligent faultdetection capabilities because deriving measurements allows for moreprecise correlation between and amongst absolute and differencesmeasures that reflect characteristics of different fault events.

At least one of the plural different determinations may comprise ameasurement relating to operation of an electrical circuit comprisingthe live and neutral conductors. For example the electrical circuit maybe a load to which electrical power is conveyed by the live and neutralconductors or a generator from which electrical power is conveyed by thelive and neutral conductors. The determination may comprise measurementof current flowing in one of the live and neutral conductors, e.g. forthe purpose of determining power consumption.

At least one of the plural different determinations may comprise adecision relating to operation of an electrical circuit comprising thelive and neutral conductors, such as load or generator to or from whichelectrical power is conveyed by the live and neutral conductors. Thedecision may comprise determining a fault condition. Normal operation ofthe electrical circuit may be interrupted in dependence on the decision.The current measurement apparatus may therefore comprise a circuitbreaker which is operative to break at least one of the live and neutralconductors in dependence on the decision. The decision may be one of anover-current decision and an arc fault decision based on at least one ofthe current measurements. For example the over-current decision maycomprise determining whether or not the current measurement for the liveconductor exceeds a threshold over-current value on average over apredetermined period of time. Alternatively or in addition and by way ofa further example the arc fault decision may comprise analysis of atleast one of the current measurements such as in respect of currentwaveform profile and comparison of the analysis with stored arc faultdata. Analysis of one of the two current measurements may provide fordetection of a series arc fault condition. Analysis of at least one ofthe current measurements may provide for detection of a parallel arcfault condition. An arc fault decision may further comprise analysingthe periodicity of arc fault events. More specifically and where thecurrent measurement apparatus comprises voltage measurement apparatus asdefined below, the arc fault decision may comprise analysing the phaseof the arc fault events relative to a phase of a voltage measurementbetween the live and neutral conductors.

The current measurement apparatus may be operative to determine adifference between the currents and one of the plural differentdeterminations may be made in dependence on the difference between thecurrents. The current difference may be determined on the basis of thedifference between digital representations of the measured currents. Thecurrent difference may be determined substantially simultaneously withthe current measurements, e.g. within comparatively few clock cyclesfollowing acquisition of the current measurements. A difference betweenthe currents in the live and neutral conductors may be indicative of anelectrical fault involving, for example, leakage of current to ground.Hence the current measurement apparatus may be operative to detect aground fault. The current measurement apparatus may further comprise acircuit breaker, which is operative to break at least one of the liveand neutral conductors in dependence on the current difference. Thecurrent measurement apparatus may therefore be operative as a GroundFault Circuit Interrupter (GFCI) or Residual Current Device (RCD). Thelevel and duration of the current difference may have a bearing on therisk presented by leaking current, e.g. to a person providing a path forthe leaking current. The current measurement apparatus may be configuredto operate the circuit breaker if the current difference exceeds athreshold value. More specifically the current measurement apparatus maycomprise a comparator, which may be implemented in digital form andwhich is operative to compare the current difference with the thresholdcurrent value. Alternatively or in addition the current measurementapparatus may operate the circuit breaker if the current difference ispresent for more than a threshold time. More specifically the currentmeasurement apparatus may comprise a time determining arrangement, suchas a timer, a counter or a filter, which is operative to determine aduration of a current difference. Alternatively or in addition thecurrent measurement apparatus may be operative to at least one of filterand non-linear process the measurements to determine a real event, suchas a person being electrocuted, from a non-event, such as a surge causedwhen a device is powered on. Alternatively or in addition, the currentmeasurement apparatus may use the absolute value of either of themeasurements to determine response of the detector to differences.Additionally if a voltage measurement is present, the nature of thedifference in current versus the voltage characteristics may be used todetermine the type of fault and whether or not to alter the thresholdsor delays, to either protect sooner or to prevent false tripping.

Alternatively or in addition the current measurement apparatus may beoperative to make at least one determination based on a differencebetween the current measurements and at least one of the currentmeasurements. The present invention may therefore be capable ofdetermining plural fault conditions, such as ground and arc faults, onthe basis of the current measurements. A prior art approach involvesrelying on a differential current transformer to provide for detectionof plural faults. More specifically a low pass filter is operative toprovide a signal which provides for ground fault detection and a highpass filter is operative to provide a signal which provides for arcfault detection. The present invention in contrast may perform suchfault detection, amongst other things, in dependence on measurement ofthe absolute currents flowing in the live and neutral conductors.

More specifically the current measurement apparatus may be configured toprocess the current difference by making a decision based on comparisonwith a changeable threshold value, the threshold value being changed independence on a level of a current measurement. For example and wherethe current measurement apparatus constitutes a GFCI, if the measuredcurrent is in the range of 10 Amps RMS to 20 Amps RMS the thresholdvalue may be set low whereas if the measured current is in the range of0 Amps RMS to 5 Amps RMS the threshold value may be set high. Where theabsolute level of current flowing in the conductors is higher a lowerlevel of leakage current may be deemed less susceptible to falsetriggering when a load is attached. In contrast another approach mayinvolve altering the threshold to provide a higher level of safety whenno load is attached. Therefore the current measurement apparatus may beoperative to change a threshold value in dependence on whether or not aload is present with presence of a load being, for example, determinedby way of at least one measurement by the current measurement apparatus.Another approach may involve at least one of: learning what thresholdvalue may be applied when a change in load occurs: and adapting thethreshold value in dependence on a change in measured value.Changeability of the threshold value may therefore provide forflexibility of operation of the current measurement apparatus. Adetermination process may be changed in dependence on at least one of: afrequency response of electronics forming at least part of the currentmeasurement apparatus; a period of analysis; a time constant ofadaptation or filtering; an absolute current measurement; and a currentdifference. Thus operation of the current measurement apparatus may bechanged so as to determine the presence or lack of a fault underdifferent conditions.

Where the current measurement apparatus is configured to compare ameasurement or a characteristic of a measurement, e.g. an absolutecurrent measurement, a current difference, a duration of a current or afrequency characteristic, with a comparative value, the currentmeasurement apparatus may be operative to change the comparative value.The comparative value may be stored in memory, such as non volatilememory, comprised in the current measurement apparatus. Storage of thecomparative value in memory means that the comparative value may be setor changed comparatively readily compared, for example, with acomparative value that is determined by hardware. Thus and in currentmeasurement apparatus on a given hardware platform a particularcomparative value may be stored upon manufacture or indeed subsequentlyupon or after deployment. For example current measurement apparatuswhich is operable to perform a RCCB function with a comparative value of30 mA RMS may be reconfigured to perform a Class A GFCI function with acomparative value of mA RMS. The current measurement apparatus maytherefore further comprise a configuration interface which is operableto set or change the comparative value. The configuration interface maybe at least one of manually operable and electronically operable. Theconfiguration interface may comprise one or more features of theconfiguration interface as described elsewhere herein.

The current measurement apparatus may be operative to change itsconfiguration in dependence on a self-learning process. Therefore acomparative value and a method of making a determination may be changedin dependence on at least one factor comprising: another measurement; ananalysis of at least one measurement, e.g. a frequency profile or phasedifference; a change in load conditions; a determination, e.g. detectionof a fault condition; at least one past measurement, e.g. average pastvalues for the same measurement. For example a particular arc event mayoccur only when a new appliance is connected. The current measurementapparatus may therefore be operative to recognise when a new applianceis connected, e.g. by way of a change in load current and the presenceof a characteristic waveform, and to categorise that particular arc asbeing of a non-dangerous form. The change in configuration may be independence solely upon operation of the current measurement apparatus,e.g. during the course of normal operation of the current measurementapparatus and without dependence on an outside agency, such as otherapparatus or manual reconfiguration. Alternatively or in addition changein configuration may be in dependence on operation of apparatus otherthan current measurement apparatus. Accordingly the current measurementapparatus may comprise a configuration interface which is operable tochange the configuration. The configuration interface may beelectronically operable. The configuration interface may comprise one ormore features of the configuration interface as described elsewhereherein. The configuration may therefore be changed in dependence onoperations at other apparatus, such as at a remote location, with theconfiguration interface providing for communication between the currentmeasurement apparatus and the other apparatus. For example a waveformacquired by the current measurement apparatus may be conveyed to theother apparatus for analysis and the other apparatus may convey controlsignals to the current measurement apparatus to change the configurationof the current measurement apparatus to perform a new determination orto change how a determination is made. Alternatively or in additionplural current measurement apparatus each comprising a configurationinterface may be operative to communicate between or amongst each otherand to make a determination based on their collective operation. Forexample if all current measurement apparatus are all operative to detecta same form of ground fault, the current measurement apparatus may becollectively operative to determine that a lightning strike hasoccurred. Each current measurement apparatus may then be operative in adifferent fashion in dependence on this determination.

Where the current measurement apparatus is configured to make pluraldifferent fault decisions, the current measurement apparatus may beconfigured to respond conditionally in dependence on detection of pluraldifferent faults at the same time. More specifically the currentmeasurement apparatus may be operative to respond to one of two detectedfaults. For example where ground and arc faults are detected the currentmeasurement apparatus may be operative to respond only to the groundfault, e.g. by operating a circuit breaker or by reporting the groundfault to a remote location. Thus priority may be given to a certain typeof fault, such as a fault liable to cause an electric shock, inpreference to other types of fault.

Where at least one of the plural different determinations comprises ameasurement relating to operation of an electrical circuit comprisingthe live and neutral conductors, such as a power consumptionmeasurement, and at least one of the plural different determinationscomprises a decision relating to operation of the electrical circuit,such as a fault condition, the current measurement apparatus may beconfigured to respond conditionally in dependence on the measurement andthe decision. For example if the measurement relates to a high level ofpower consumption no action may be taken, e.g. by way of operation of acircuit breaker, until a period of time has elapsed. Thus the likelihoodof false circuit breaker operation may be reduced.

The current measurement apparatus may be configured to determine afrequency characteristic of at least one measurement. More specificallythe current measurement apparatus may be operative to respond independence on a determined frequency characteristic. A measurement, suchas of an absolute current in the live conductor, may comprise at leastone frequency component, which is indicative of an operative conditionof an electrical circuit electrically connected to the live and neutralconductors. More specifically the operative condition may be indicativeof one of normal operation or faulty operation. A normal operativecondition may be the switching on or off of the electrical circuit, theoperation of a motor comprised in the electrical circuit or the like.Having an indication of such normal operative conditions may allow for adecision to be taken not to take certain action, e.g. involvingoperation of a circuit breaker, when a fault detection operation asdescribed elsewhere may be liable to detect a fault conditionincorrectly. A faulty operative condition, e.g. as reflected by acertain frequency profile, may be indicative of an arc or ground fault.Having an indication of a faulty operative condition may allow for adecision to be taken on the basis of such an indication as well as independence on operation of a fault detection operation as describedelsewhere.

Alternatively or in addition the plural different determinations maycomprise at least two of: series arc fault detection; parallel arc faultdetection; ground fault detection; current measurement, e.g. formetering purposes; and over-current detection. The current measurementapparatus may therefore be configured to make a subset of suchdeterminations. Accordingly the current measurement apparatus mayfurther comprise a configuration interface which is operable toconfigure the current measurement apparatus to make selecteddeterminations. The configuration interface may be at least one offirmware configurable, hardware configurable manually operable andelectronically operable. Where the configuration interface is firmwareconfigurable reconfiguration may be achieved by changing the code usedby the apparatus, for example by changing the settings in non volatilememory on the PCB of the current measurement apparatus. Where theconfiguration interface is hardware configurable reconfiguration may beachieved by a link or component present on a PCB of the currentmeasurement apparatus. Where the configuration interface is manuallyoperable the configuration interface may comprise a manually operablecontrol, such as DIP switches. Where the configuration interface iselectronically operable the configuration interface may comprise anelectronic interface which is configured to receive electronicconfiguration signals, e.g. locally from a Personal Computer by way of awired or wireless communications link or remotely from a locationforming part of a distribution network by way of a wired or wirelesscommunications channel. Configuration may, for example, be at deploymentof the current measurement apparatus and depend on the circumstances ofuse. Alternatively a configuration may be changed after deployment, e.g.locally or from a remote location, to take account of changing usagerequirements or a change in regulatory requirements, such as astipulation that series and parallel arc faults must be detected in allresidential rooms. More specifically a first current measurementapparatus may be configured to make arc fault and ground faultdeterminations where the load is in a bedroom, a second currentmeasurement apparatus may be configured to make a ground faultdetermination where the load is in a bathroom and a third currentmeasurement apparatus may be configured to make an arc faultdetermination where the load is in a living room, with each of the firstto third current measurement apparatus being configured in addition tomeasure current for metering requirements and to provide over currentprotection. In addition each of the first to third current measurementapparatus may be configured to provide for a different level of overcurrent protection, e.g. by way of different threshold values, with eachthreshold value stored in memory, such as in non volatile memory. Pluralcurrent measurement apparatus may, for example, be installed in adistribution box or the like with each current measurement apparatusbeing configured to carry out different determinations despite all thecurrent measurement apparatus being based on the same hardware platformand substantially the same firmware platform. Furthermore each currentmeasurement apparatus may be reconfigurable to take account of changingrequirements, as described above.

The current measurement apparatus may comprise a configuration interfacewhich is operable to change a configuration of the current measurementapparatus. The configuration interface may comprise one or more of thefeatures described above. The configuration interface may be operable tochange the configuration so as to provide a hitherto un-provided meansof providing a determination. For example the configuration interfacemay be used to change the configuration of the current measurementapparatus so that a fault, such as an arc fault, is detected on thebasis of different measured signals or by analysing the measured signalsin a different fashion.

At least one of the first and second measurement devices may comprise anelectrical component in series with a load which is electricallyconnected to the conductor. The electrical component may comprise ashunt resistor. At least one of the first and second measurement devicesmay comprise an electrical circuit disposed in relation to theconductor, the electrical circuit being configured such that a currentsignal in the conductor induces an electrical signal, such as a currentsignal, in the electrical circuit. More specifically the electricalcircuit may be one of a current transformer, a Rogowski coil and a Halleffect sensor.

The current measurement apparatus may further comprise voltagemeasurement apparatus, which is configured to measure a voltage betweenthe live and neutral conductors. The voltage measurement apparatus maycomprise a potential divider electrically connected between the live andneutral conductors. The current measurement apparatus may be configuredto respond in dependence on a voltage measurement by the voltagemeasurement apparatus. More specifically the current measurementapparatus may be operative to determine power consumption in dependenceon current measurement and voltage measurement. Alternatively or inaddition the current measurement apparatus may be operative to respondconditionally in dependence in part on a voltage measurement. Forexample and where the current measurement apparatus is operative todetect an arc fault the current measurement apparatus may not beoperative to respond to the detected arc fault, e.g. by way of operationof a circuit breaker, in dependence on a characteristic of the voltagesignal, such as a peak in voltage signal indicative of normal circuitoperation rather than an arc fault condition. The voltage measurementapparatus may be operative to determine simultaneously more than onedetermination, for example determinations used for both powermeasurement and arc fault detection. Alternatively or in additional theapparatus may be operative to determine a fault without being operativeto interrupt supply. The apparatus may be further be operative toprovide an indication of the fault, e.g. by way of a display or remotecommunications apparatus comprises in the apparatus. For example theapparatus may be comprised in a smart meter which is operable tosimultaneously measure power using the absolute current and voltagemeasurements and detect faults using derived difference measurements.Such a smart meter may, for example, be further operative to inform theutility if the smart meter has detected a ground fault in a property inwhich the smart meter is installed.

Responding to different circuit conditions may comprise measuringsignals within different dynamic ranges. Furthermore such signals mayneed to be measured to high accuracy. For example a first circuitcondition may involve leakage of ground current when the absolutecurrents are within a range of 0 to 20 Amps RMS and require that theresolution and gain matching is better than 1 mA and a second circuitcondition may involve arcing within a range of 50 to 200 Amps RMS.Measurement of signals within disparate dynamic ranges with signalacquisition circuitry of fixed dynamic range may result in one of thesignals being measured to insufficient accuracy. For example an arcingsignal in the 50 to 200 Amps RMS range may be measured to sufficientaccuracy whereas a leakage signal in the 0 to 20 Amps RMS range may bemeasured to insufficient accuracy. An approach may involve providing ameasurement device for each of the different measurement requirementsand signal conditioning and acquisition circuitry for each measurementdevice. For example a current transformer may be disposed around thelive conductor and a shunt resistor may be connected in series with thelive conductor, with the current transformer and its associatedcircuitry being configured to measure large amplitude signals, e.g. forarc fault detection, and the shunt resistor and its associated circuitrybeing configured to measure low amplitude signals, e.g. for powerconsumption measurement. The present inventors have appreciated thisapproach to involve circuit complexity. Also this approach may presentdifficulties in making full use of measured signals, e.g. with regardsto more sophisticated determinations such as those described above, inparticular where absolute current measurements are made on both live andneutral conductors. The inventors have therefore devised an improvement.

In accordance with the improvement the current measurement apparatus mayfurther comprise at least one acquisition circuit, the at least oneacquisition circuit being configured to have at least two differentdynamic ranges and to be operative to acquire signals from a measurementdevice within a respective one of the different dynamic ranges. Forexample a first signal may be acquired when the at least one acquisitioncircuit is operative within a first dynamic range and a second signalmay be acquired when the at least one acquisition circuit is operativewithin a second dynamic range with both the first and second signalsbeing acquired from the same measurement device. The at least oneacquisition circuit may be configured for progressive change of dynamicrange. The current measurement apparatus may comprise at least oneacquisition circuit, which is operative to acquire signals from thefirst measurement device and at least one acquisition circuit, which isoperative to acquire signals from the second measurement device, each ofthe at least one acquisition circuit being configured as describedabove.

The at least one acquisition circuit may comprise at least a first and asecond analogue to digital converter, the first analogue to digitalconverter being configured to acquire a signal within a first dynamicrange and the second analogue to digital converter being configured toacquire a signal within a second, different dynamic range. At least oneof the first and second first analogue to digital converters maycomprise a gain circuit which is operative to amplify or attenuate ananalogue signal from the measurement device before conversion of theanalogue signal to a digital signal. Thus for example a first gain stagemay amplify an analogue signal by a first predetermined amount beforeconversion to a digital form by the first analogue to digital converterand a second gain stage may amplify an analogue signal by a secondpredetermined amount before conversion to a digital form by the secondanalogue to digital converter. Additionally the current measurementapparatus may be operative determine the dynamic ranges of eachacquisition circuit by comparing the histograms of each path when thesignal is within both dynamic ranges and using the comparisoninformation to normalise the gain of each channel to provide forconsistency.

Alternatively or in addition the at least one acquisition circuit maycomprise an analogue to digital converter comprising an adjustable gaincircuit, the adjustable gain circuit being configured to amplify orattenuate an analogue signal from the measurement device by one ofplural different amounts before conversion of the analogue signal to adigital signal. The at least one acquisition circuit may furthercomprise a dynamic range detector circuit, which receives an output fromthe analogue to digital converter and is operative in dependence thereonto select one of a plurality of dynamic ranges in dependence on anamplitude of the output from the analogue to digital converter. The atleast one acquisition circuit may further comprise a gain controllerwhich is operative in dependence on an output from the dynamic rangedetector circuit to change a gain of the adjustable gain circuit.

Measurement of signals having different frequency profiles may presentproblems with regards to accuracy. For example a leakage current maycomprise frequency components of up to 1 kHz whereas arcing signals maycomprise frequency components in the 100 kHz range. Accurate measurementin different frequency bands may result in either inaccuracy where thesignal acquisition circuitry is unable to respond to high frequencysignals or over sampling of low frequency signals where the signalacquisition circuitry is capable of responding to high frequencysignals, e.g. by being clocked at a rate commensurate with the highfrequency signals. For example a current transformer may be disposedaround the live conductor and a shunt resistor may be connected inseries with the live conductor, with the current transformer and itsassociated circuitry being configured to measure high frequency signals,e.g. for arc fault detection, and the shunt resistor and its associatedcircuitry being configured to measure low frequency signals, e.g. forpower consumption measurement. The current measurement apparatus maytherefore comprise plural acquisition circuits, the plural acquisitioncircuits being configured to acquire signals from a measurement deviceat different sampling rates. For example a first acquisition circuit maybe operative to sample a signal at 10 kHz and a second acquisitioncircuit may be operative to sample a signal at 1 MHz. More specificallythe plural acquisition circuits may be configured to be clocked atdifferent frequencies.

According to a second aspect of the present invention there may beprovided a current measurement arrangement comprising plural currentmeasurement apparatus according to the first aspect of the presentinvention, each of the plural current measurement apparatus beingconfigured to measure current in a different one of plural liveconductors and a neutral conductor. Thus the current measurementarrangement may be operative to measure current in different phases of athree phase electrical supply. Embodiments of the second aspect of theinvention may comprise one or more features of the first aspect of theinvention.

According to a third aspect of the present invention there is provided acurrent measurement method comprising: operating first and secondmeasurement devices to measure current in a respective one of a liveconductor and a neutral conductor substantially simultaneously; andoperating current measurement apparatus to make plural differentdeterminations in dependence on the substantially simultaneous currentmeasurements.

Embodiments of the third aspect of the present invention may compriseone or more features of the first or second aspect of the presentinvention.

According to a fourth aspect of the present invention there is providedelectrical apparatus comprising current measurement apparatus accordingto the first aspect of the present invention or a current measurementarrangement according to the second aspect, the electrical apparatusbeing configured such that the current measurement apparatus or currentmeasurement arrangement measures current passing through a part of theelectrical apparatus.

The electrical apparatus may be a socket, a plug or electrical adapter.Alternatively or in addition the electrical apparatus may compriseelectricity generation, transmission or distribution apparatus. Theelectrical apparatus may, for example, be constituted by an electricitymeter or a distribution box with the current measurement apparatus beingoperative to measure current passing through the electricity meter ordistribution box. The current measurement apparatus may thereby providea means to measure the electricity consumption and to detect faultconditions and respond accordingly.

Alternatively or in addition the electrical apparatus may compriseelectrical propulsion apparatus comprising an electrical energy storageor generation device, such as a battery or fuel cell. The electricalpropulsion apparatus may be configured such that the current measurementapparatus is operative to provide for control, e.g. shut down, in theevent of a fault condition and regulation of at least one of: powersourced by the electrical energy storage or generation device; and powersunk by the electrical energy storage device, e.g. during charging. Safeand reliable delivery of electrical power to electric motors at highcurrent levels is normally required of such electrical propulsionapparatus. Accurate current measurement may therefore be required toprovide for proper regulation and control and to respond to faultconditions.

Alternatively or in addition the electrical apparatus may compriseelectrical control apparatus comprising an electrical actuator. Theelectrical control apparatus may be configured such that the currentmeasurement apparatus is operative to measure current drawn by theelectrical actuator. The electrical actuator may comprise a motor andthe current measurement apparatus may be comprised in a motor controllerwhich is operative to control the motor. Electrical control apparatusmay be used in diverse fields, such as manufacturing, commercialmachinery and process control. For example the electrical actuator maycomprise a stepper motor forming part of a CNC machine or driving avalve in a fluid control system. Alternatively the electrical actuatormay comprise a linear solenoid in an electrically controlled automotivetransmission system. In such applications accurate measurement ofcurrent may provide for precision of control and for response to faultconditions.

Further embodiments of the fourth aspect of the present invention maycomprise one or more features of any previous aspect of the presentinvention.

Further embodiments of any one of the first to fourth aspect of thepresent invention may comprise one or more features of any other aspectof the present invention, in particular but not exclusively to featuresof the fifth aspect of the present invention, such as features relatingto calibration.

According to a fifth aspect of the present invention there is providedcurrent measurement apparatus comprising first and second measurementdevices, each of the first and second measurement devices beingoperative to measure current in a respective one of a live conductor anda neutral conductor, the current measurement apparatus being operativeto determine a difference between the measured currents and to make adetermination in dependence on the current difference.

Prior art approaches involve measurement of a difference between thecurrents flowing in the live and neutral conductors. In contrast thepresent invention involves measuring the absolute current flowing ineach of the live and neutral conductors and determining the currentdifference based on the absolute current measurements.

More specifically the current measurement apparatus may be configuredfor ground fault detection. The current measurement apparatus maytherefore be operative to compare the current difference with athreshold leakage value, such as 30 mA, and if the current differenceexceeds the threshold leakage value make a ground fault determination.The current measurement apparatus may further comprise a circuitbreaker, which is configured to break at least one of the live andneutral conductors. The circuit breaker may be operative in dependenceon a ground fault determination being made.

Measurement of absolute current flowing in each of the live and neutralconductors may be liable to inaccuracy. Also a high level of relativeaccuracy may be required between current measurements made on the liveand neutral conductors to obtain a current difference to high accuracy.An acceptable level of inaccuracy for each absolute current measurementmay be insufficient to obtain a current difference to required accuracyin particular when the absolute current signals are large. The currentmeasurement apparatus may therefore be configured to provide forcalibration of at least one of the first and second measurement devices.More specifically the current measurement apparatus may be configured toprovide for calibration of both of the first and second measurementdevices. The current measurement apparatus may be configured to providefor calibration on a periodic basis. Alternatively or in addition thecurrent measurement apparatus may be configured to provide forcalibration in dependence on a state change of the current measurementapparatus, such as when the current measurement apparatus is powered up.Alternatively or in addition current measurement apparatus may beoperative to remove the calibration signal from the measurement beforeany analysis to prevent the calibration signal from giving rise to anerror in fault detection or power measurement.

According to one approach the current measurement apparatus may beconfigured to apply a calibration signal to at least one of the firstand second measurement devices. More specifically at least onecalibration signal may be applied to both the first and secondmeasurement devices. The first and second measurement devices may beoperative to measure the applied calibration signal. Therefore the firstand second measurement devices may be calibrated in view of thecalibration signal being known or predetermined. The calibration signalmay only need to be substantially the same for the first and secondmeasurement devices to be sufficient to allow matching of thecharacteristics of the two measurement devices by determination of acorrection factor to be applied, using the correlation of the extractedcalibration signals, without needing to know the absolute accuracy ofthe calibration signal. The current measurement apparatus may be subjectto at least one of the following calibration procedures in addition toor instead of calibration upon manufacture or assembly.

In a first form a calibration signal may be passed through a measurementdevice. This embodiment may be appropriate where the measurement devicecomprises an electrical component, such as a shunt resistor, in serieswith a load electrically coupled to the live and neutral conductors.Thus, for example, the calibration signal may be applied to at least oneof the live and neutral conductors such that the calibration signalpasses through the measurement device, whereby the measurement device isoperative to measure the calibration signal. The current measurementapparatus may further comprise a calibration source, which is operativeto apply the calibration signal. The calibration source may compriseactive and passive components. Furthermore, the calibration source maycomprise an impedance in series with at least one switch, the seriesconnected impedance and at least one switch being connected between thelive and neutral conductors. In use the impedance and the at least oneswitch may be connected between the live and neutral conductors on asame side of the first and second measurement devices as a load. Theimpedance may comprise at least one of a resistor, an inductor and acapacitor. Use of a capacitor may be advantageous because substantiallyno active power is dissipated by the capacitor and is therefore capableof delivering more current at less cost and a lower power consumptionthan, for example, a resistor. The calibration source may furthercomprise a controller, which is operative to open and close the at leastone switch. Opening and closing the at least one switch in apredetermined fashion may apply a predetermined calibration signal toboth the live and neutral conductors.

A switch which is operative to close and thereby connect the impedancebetween the live and neutral conductors may be required to withstand ahigh voltage when open. In many applications the line voltage betweenthe live and neutral conductors may be at mains levels and may behundreds of volts in certain applications. Withstanding such high offvoltages may be problematic for a switch and in particular for a switchformed in an integrated circuit. The present inventors have devised animprovement in the light of an appreciation of this problem. Accordingto the improvement the current measurement apparatus may comprise animpedance and plural switches which are operative such that there isalways a circuit path, which comprises at least one closed one of theplural switches, between the live and neutral conductors. Morespecifically the current measurement apparatus may comprise fourswitches and may be configured such that two of the four switches arealways closed. During operation there may never be solely one signalpath between the live and neutral conductors comprising the impedanceand an open switch. A maximum voltage across a switch may therefore bemuch lower than according to a configuration in which an open switchforms part of solely one circuit path between the live and neutralconductors.

Where the impedance comprises a capacitor the current measurementapparatus may further comprise a voltage source in series with thecapacitor between the live and neutral conductors. The voltage sourcemay be operative to apply a changing voltage signal, such as a sinusoid,between the capacitor and one of the live and neutral conductors. Thechanging voltage signal may have a frequency component higher than afundamental frequency of the line voltage. The current measurementapparatus may further comprise a resistor in series with the capacitorand the voltage source, the resistor being operative to sense a currentsignal, i.e. a calibration signal, in a measurement device. The currentmeasurement apparatus may yet further comprise a measurementconfiguration which is operative to measure a current signal in theresistor. The measurement configuration may, for example, comprise asample and hold circuit and analogue to digital converter which areoperative to measure a voltage signal across the resistor. The currentmeasurement apparatus may be configured to alter the changing voltagesignal applied by the voltage source in dependence on at least onemeasurement made by the measurement configuration. The currentmeasurement apparatus may therefore be operative to control thecalibration signal applied to at least one of the measurement devices.The current measurement apparatus may further comprise an inductor. Theinductor may be operative to store and release calibration signalcurrent whereby the calibration signal is spread over a longer timeperiod which may be more in line with the frequency capabilities ofacquisition circuitry. In addition or alternatively the calibrationsignal may be replicated locally with a known multiplication factoracross the measurement device to more power efficiently replicate thecalibration signal on the other conductor.

The present inventors have appreciated that the amplitude of thecalibration signal generated by a passive impedance normally follows thephase of the line voltage, i.e. the phase of the voltage between thelive and neutral conductors. For example if the impedance is capacitivethe amplitude of the calibration signal may be at a maximum when therate of change of the line voltage is at a maximum, i.e. at the zerocrossing point of the line voltage. On the other hand, and by way offurther example, if the impedance is resistive the amplitude of thecalibration signal may be at a maximum when the line voltage is at amaximum. The amplitude of the line voltage affects the signal to noiseratio (SNR). Irrespective of whether the impedance is capacitive orresistive the rms signal to rms noise ratio is obtained if one averagesall measurements from a measurement device over at least one completecycle of the line voltage. However the SNR varies from measurement tomeasurement from a measurement device within a complete cycle of theline voltage. In view of this the current measurement apparatus may beconfigured where the impedance is substantially resistive to weightmeasurements from a measurement device in dependence on the line voltagesignal. Where the impedance is reactive the current measurementapparatus may be configured to weight measurements differently within acycle of the line voltage. More specifically there may be a progressivechange, i.e. increase or decrease, in weighting of a series ofmeasurements. Alternatively or in addition a weighting profile maycorrespond to a profile of the line voltage signal. The weighting ofmeasurements with better SNR in preference to measurements with poorerSNR within a cycle may improve overall SNR. Alternatively andadditionally the current measurement apparatus may be operative toanalyse noise locked to the line frequency and choose to process theacquired signals appropriately, for example by throwing awaymeasurements that might be erroneous. Alternatively or additionally thecurrent measurement apparatus may be operative to change the frequency,phase, amplitude or modulation of the calibration signal to improve theacquired SNR.

The current measurement apparatus may be configured such that the pluralswitches are operative to alternately connect the impedance betweenfirst and second opposing ends of each of the first and secondmeasurement devices. More specifically the plural switches may beoperative to connect the impedance between one of the first and secondends of the first measurement device and one of the first and secondends of the second measurement device. In use the first end of each ofthe first and second measurement devices may be electrically coupled toa source and the second end of each of the first and second measurementdevices may be electrically coupled to a load. More specifically theplural switches may be operative to connect the impedance to the firstends of the first and second measurement devices at one time, e.g.during a first phase, and to connect the impedance to the second ends ofthe first and second measurement devices at another time, e.g. during asecond phase. During the first phase no calibration signal may passthrough the first and second measurement devices and during the secondphase the calibration signal may pass through the first and secondmeasurement devices. According to this approach a maximum voltage seenby the switches is the voltage across a measurement device which isliable to be in the tens of mV range. Alternatively or in addition andin use, the first end of the first measurement device and the second endof the second measurement device may be electrically coupled to a sourceand the second end of the first measurement device and the first end ofthe second measurement device may be electrically coupled to a load.More specifically the plural switches may be operative as describedabove such that the impedance is connected to the first ends of thefirst and second measurement devices during a first phase and to thesecond ends of the first and second measurement devices during a secondphase. During the first phase a calibration signal may pass through thesecond measurement device but not through the first measurement deviceand during the second phase a calibration signal may pass through thefirst measurement device but not through the second measurement device.

The present inventors have appreciated that having a configuration inwhich plural switches alternately connect the impedance between firstand second opposing ends of each of the first and second measurementdevices may provide for accurate calibration of the transfer function ofthe first and second measurement devices and may provide for reductionif not removal of a signal with characteristics common to thecalibration signal and the live and neutral conductors, i.e. a commonsignal. Where the plural switches are operative such that no calibrationsignal and a calibration signal pass through the first and secondmeasurement devices during the first and second phases respectively, thecurrent measurement apparatus may be operative to subtract measurementsmade by the first and second measurement devices from one another duringone of the first and second phases and measurements made by the firstand second measurement devices from one another during the other of thefirst and second phases. The current measurement apparatus may thereforebe operative to determine a difference between measurements with thecalibration signal and also measurements lacking the calibration signalon a time spaced basis. The current measurement apparatus may beoperative to correlate the measurements to be subtracted with each otherprior to subtraction. The current measurement apparatus may thereforeprovide for removal of the common signal and determination of thecalibration signal. The determined calibration signal may then be usedas described elsewhere to determine a normalisation factor to be appliedto measurements, for example, before the measurements are used for atleast one of fault detection and power measurement. The calibrationsignal on the live conductor may be removed by determining the averageof Ical(live)=Ilive(phase 2)−Ilive(phase 1) where Ical(live) is thecalibration signal present on the live conductor, IIlive(phase 2) is thelive current measured during phase 2 and Ilive(phase 1) is the livecurrent measured during phase 1. Similarly calibration signal on theneutral conductor may be removed by determining the average ofIcal(neutral)=Ineutral(phase 2)−Ineutral(phase 1) where Ical(neutral) isthe calibration signal present on the neutral conductor, Ineutral(phase2) is the neutral current measured during phase 2 and Ineutral(phase 1)is the neutral current measured during phase 1. The error in a gainmismatch, A, between the live and neutral conductors may thereby bedetermined. After application of the gain mismatch to all subsequentmeasurements and removal of the calibration signal the differencebetween the live and neutral current signals may be determined. Howeverthis approach has the disadvantage of giving rise to an error wherethere is common signal having the same characteristics as thecalibration signal. A further limitation may arise when the live currentsignal shares characteristics with the calibration signal the currentmeasurement apparatus may be unable to distinguish the calibrationsignal from the live current signal. Alternatively or in addition thecurrent measurement apparatus may be operative to subtract measurementsmade by the first and second measurement devices from one another duringone of the first and second phases and measurements made by the firstand second measurement devices from one another during the other of thefirst and second phases when the plural switches are operative such thatthe calibration signal passes through one of the first and secondmeasurement devices during the first phase and the calibration signalpasses through the other of the first and second measurement devicesduring the second phase. According to this approach, a differenceobtained during the first phase is of the form ((Signal+Ical)−A*Signal)and a difference obtained during the second phase is of the form(Signal−A*(Signal+Ical)), where Signal is the load current signal, Icalis the calibration signal and A is the gain mismatch between the liveand neutral conductors. The current measurement apparatus may be furtheroperative to subtract the two differences from one another to therebydetermine a factor in A and Ical. Alternatively or additionally theapparatus may be operative to use an iterative approach over time on thelive and neutral measurements to firstly determine an initial value ofA, which is then used to extract an initial value of Ical, which is inturn used to estimate a better value of A, and to thereby determine anoverall accurate value. The factor may therefore take account of thegain mismatch between the live and neutral conductors. This approach mayprovide for removal of the common signal subject to the common signalbeing substantially the same on average over the first and secondphases.

The present inventors have appreciated that the requirement for thecommon signal being substantially the same on average over the first andsecond phases may be obviated by changing the configuration of thecurrent measurement apparatus. The current measurement apparatus maytherefore further comprise third and fourth measurement devices, thethird measurement device being disposed in series with the firstmeasurement device in the live conductor and the fourth measurementdevice being disposed in series with the second measurement device inthe live conductor. The calibration source may be configured such thatthe calibration signal is applied to only the first and secondmeasurement devices. For example and where the calibration sourcecomprises an impedance and at least one switch the impedance may beoperative to periodically couple the live conductor between the firstand third measurement devices to the neutral conductor between thesecond and fourth measurement devices. This approach may provide forreduction if not removal of a signal with characteristics common to thecalibration signal and the live and neutral conductors, i.e. a commonsignal. The current measurement apparatus may be configured to determinea difference between signals measured by the first and secondmeasurement devices, a difference between signals measured by the thirdand fourth measurement devices and a difference between the thusdetermined differences. Alternatively or in addition the currentmeasurement apparatus may be configured to determine a differencebetween signals measured by the first and third measurement devices, adifference between signals measured by the second and fourth measurementdevices and a difference between the thus determined differences. Thecurrent measurement apparatus may be configured to extract thecalibration signal from the measured signals as described herein below.The current measurement apparatus may be configured to at least one ofcorrelate measurements and normalise measurements. More specifically thecurrent measurement apparatus may comprise digital processing circuitrywhich is configured to at least one of: correlate measurements with eachor one another; normalise measurements with each other or one another;apply calibration factors to measurements; control application of acalibration signal; and extract a calibration signal from measuredsignals. Alternatively or in addition the current measurement apparatusmay be configured to combine measurements made with the first to fourthmeasurement devices to thereby improve upon the SNR. More specificallythe current measurement apparatus may be operative to evaluateIlive(cal)=I1−alpha*I3, where alpha is the normalised gain error betweenI1 and I3, and Ineutral(cal)=I2−beta*I4, where beta is the normalisedgain error between 12 and I4 and where I1, I2, I3 and I4 are themeasured currents in the first to fourth shunt resistors respectively.The current measurement apparatus may be further operative to compareIlive(cal) and Ineutral(cal) to determine the gain error between I1 andI2.

In a second form a calibration signal may be passed through acalibration conductor, the calibration conductor being disposedproximate the measurement device and the measurement device beingconfigured such that the calibration signal induces a correspondinginduced calibration signal in the measurement device. This embodimentmay be appropriate where the measurement device comprises an inductivecurrent sensor, such as a current transformer or a Rogowski coil.Therefore the measurement device may comprise a coil which is disposedaround the calibration conductor.

The calibration conductor may be disposed proximate both the firstmeasurement device and the second measurement device. The currentmeasurement apparatus may therefore be operative to apply a calibrationsignal to the calibration conductor, whereby the calibration signalpasses proximate the first measurement device and the second measurementdevice in turn.

The current measurement apparatus may comprise first and secondcalibration conductors, which are disposed proximate a respective one ofthe first and second measurement devices. A proximal end of each of thefirst and second calibration conductors may be electrically coupled tothe calibration source. A distal end of each of the first and secondcalibration conductors may be electrically coupled to the neutralconductor. The calibration source may be operative to apply first andsecond calibration signals to a respective one of the first and secondcalibration conductors. The first and second calibration signals may bethe same calibration signal. The current measurement apparatus maytherefore comprise a splitter which is operative, for example, to applythe same calibration signal at the same time to the first and secondcalibration conductors. Alternatively or in addition the currentmeasurement apparatus may comprise a switch which is operative to applythe same calibration signal at different times to the first and secondcalibration conductors. The first and second calibration signals may bedifferent calibration signals. The application of different calibrationsignals to the first and second measurement devices may be appropriatewhere the measurement devices have different characteristics. Forexample the first and second measurement devices may be of a differentform, such as a shunt resistor or a current transformer, or the firstand second measurement devices may be of the same form but differentconfiguration, such as two current transformers configured to measuredifferent ranges of current. The current measurement apparatus may beconfigured appropriately, for example by way of a switch which isoperative to apply the different calibration signals in turn to thefirst and second calibration conductors.

The first and second forms of calibration may be comprised in thecurrent measurement apparatus. For example the current measurementapparatus may comprise a shunt impedance and an inductive sensor, eachof which is operative to measure current in a respective one of the liveand neutral conductors, the calibration conductor passing through thecoil of the inductive and being connected thereafter to the other of thelive and neutral conductors. Thus a calibration signal applied to thecalibration conductor induces a corresponding induced current signal inthe inductive sensor before passing through the impedance sensor by wayof the conductor.

According to either the first or second form of calibration, thecalibration signal may be of predetermined form to provide for ease ofextraction of the calibration signal from measurements made by themeasurement device. More specifically the calibration signal maycomprise at least one predetermined frequency component with theextraction being dependent on the at least one predetermined frequencycomponent.

Calibration may be provided for according to another approach bymeasuring the current in the live and neutral conductors over a periodof time during normal operation, storing the measurements andsubsequently using the stored measurements to effect calibration.Therefore the current measurement apparatus may be configured: tooperate the first and second measurement devices to measure current inthe live and neutral conductors; to store the measured currents; and toadjust subsequent current measurements in dependence on the storedmeasured currents. Current measurement for calibration purposes maypreferably be carried out to reflect normal operation of an electricalcircuit comprising the live and neutral conductors, e.g. such thatmeasurement of fluctuations in current caused by noise, switchingtransients and the like are reduced. The current measurement apparatusmay therefore be operative to make plural current measurements over aperiod of time, such as over a period of minutes or hours, and todetermine a current measurement value for storage in dependence on theplural current measurements, e.g. an average value for the pluralcurrent measurements.

The current measurement apparatus may be configured to determine amatching function which is operative to bring a response of the firstmeasurement device and a response of the second measurement device intoproximity with each other. Bringing the responses into proximity witheach other may provide for improved accuracy of current differencedetermination, e.g. by reducing an offset between outputs from the twomeasurement devices. More specifically the matching function may beoperative to bring a transfer function of the first measurement deviceand a transfer function of the second measurement device into proximitywith each other. Bringing the transfer functions into proximity witheach other may provide for improved accuracy of current differencedetermination, e.g. by changing at least one of the transfer functionsso as to bring the transfer functions into proximity with each other.The current measurement apparatus may be configured to apply, e.g. byway of the calibration source, plural different current values to thefirst and second measurement devices. In addition the currentmeasurement apparatus may be configured to determine, e.g. by way ofprocessing circuitry, transfer function measurements made by themeasurement devices corresponding to the applied current values. Thecurrent measurement apparatus may be operative to determine the transferfunctions in dependence on the transfer function measurements. At leastthree transfer function measurements may be made where the transferfunction has the form of a polynomial. The current measurement apparatusmay be configured to determine first and second matching functions, thefirst matching function being operative when applied to bring a transferfunction for the first measurement device into proximity with an objecttransfer function and the second matching function being operative whenapplied to bring a transfer function for the second measurement deviceinto proximity with the object transfer function. The object transferfunction may be of a form which provides for ease of processing. Forexample the object transfer function may be linear. Alternatively thefirst and second matching functions may be operative when applied totheir respective transfer functions to linearise and bring the transferfunctions into proximity with each other. Where the current measurementapparatus comprises an electronically operated interrupter or circuitbreaker which is operative to disconnect the load under a determinedfault condition, the current measurement apparatus may be operative toapply a delay between making a determination to arm the breaker andclosing the interrupter or circuit breaker to thereby re-connect thesupply. This allows time for the current measurement apparatus tocalibrate the two measurement paths to a required accuracy. Inadditional or alternatively the current measurement apparatus maycomprise sensing circuitry to monitor voltage signals on each side ofthe interrupter or circuit breaker to determine if the interrupter orcircuit breaker should be closed. The sensing circuitry may be operativeto detect a miss-wiring or to determine optimum timing for theinterrupter to be opened or closed, e.g. at a zero-crossing point of themains cycle.

The current measurement apparatus may further comprise at least oneanalogue to digital conversion apparatus which is operative to receivean analogue signal from at least one measurement device and generate adigital signal corresponding to the analogue signal. The at least oneanalogue to digital conversion apparatus may be configured for at leastone of: selection of one dynamic range from plural dynamic ranges;selection of one precision from plural precisions; and selection of onefrequency of operation from plural frequencies of operation, e.g. inrespect of bandwidth and clocking rate. The current measurementapparatus may therefore be capable of responding properly andappropriately to a variety of fault conditions and in particular arcfault conditions. The current measurement apparatus may comprise atleast one isolation circuit which is disposed so as to maintain galvanicisolation between at least two parts of the current measurementapparatus. For example an isolation circuit may be operative to maintainisolation between a first part of the current measurement apparatus,which is operative with a first measurement device in the form of ashunt resistor, and a second part of the current measurement apparatus,which is operative with a second measurement device in the form of ashunt resistor. A variety of levels of information may be passed acrossthe isolation barrier. The information may comprise the raw data fromthe analogue-to-digital converter, pre-processed data that has beenadjusted in at least one of offset, gain, phase, frequency and samplingrate and statistical information over a period of time, amongst otherthings.

The current measurement apparatus may comprise digital processingcircuitry which is operative on digital signals corresponding to orbased on current measurements. The digital processing circuitry may, forexample, comprise digital signal processing circuitry. The digitalprocessing circuitry may be configured to at least one of: correlate acurrent measurement from the first measurement device with a currentmeasurement from the second measurement device; bring responses of thefirst and second measurement devices into proximity with each other;apply predetermined calibration factors to measured signals; controlapplication of a calibration signal; and extract a calibration signalfrom a measured signal. The digital processing circuitry may beconfigured to at least one of perform computations, e.g. with regards topower consumption determination, over-current detection or arc signalrecognition, and control operations, such as of a circuit breaker orstatus indicator.

Measurement of the line current by way of a single measurement device inaddition to determining the difference between the live and neutralcurrents by way of the first and second measurement devices may conferbenefits. For example this approach may provide for ease of at least oneof simultaneous power measurement and fault detection, an AFCI functionover different voltage and frequency ranges, and enhancements to groundfault detection. This approach may also provide for enhancement ofdetection of ground faults by determining if a leakage current is activeor reactive and in dependence on this distinguishing a false trippingmechanism from a true fault.

The use of absolute current measurements on live and neutral to create adifference signal for fault detection can have significant advantagesfor correlation to absolute measurements and determination of multiplefactors with different frequency and amplitude characteristics, such assimultaneous power measurement and gfci and afci, but it may havelimited accuracy at certain amplitudes of current signals. The currentmeasurement apparatus may therefore further comprise a differentialmeasurement device which is configured to measure of itself the sum incurrent in the live and neutral conductors. The differential measurementdevice may be configured to be disposed relative the live and neutralconductors to measure the sum of the current signals present in the liveand neutral conductors. The differential measurement device may, forexample, be a differencing current transformer through which the liveand neutral conductors pass. Where the current measurement apparatuscomprises a calibration source, the calibration source may be operativeto apply a calibration signal to the differential measurement device.The applied calibration signal may be extracted from measurements asdescribed elsewhere herein to provide for calibration of thedifferential measurement device in addition to at least one of the firstand second measurement devices. The differential measurement device maybe configured to be disposed relative the live and neutral conductorssuch that no calibration signal passes through the portion of live andneutral conductor sensed by the differential measurement device. Morespecifically the differential measurement device may be configured to bedisposed on a load side of the calibration source. In such aconfiguration and where the differential measurement device isinductive, the calibration source may be configured to pass thecalibration signal through a calibration conductor, the calibrationconductor being disposed proximate the differential measurement deviceto thereby induce a corresponding induced calibration signal in thedifferential measurement device as described elsewhere herein. For sucha configuration the calibration source need not be configured to be highvoltage capable. Alternatively or in addition and where the differentialmeasurement device is inductive the current measurement apparatus may beconfigured to pass the calibration signal through one of the part of thelive and neutral conductors proximate the differential measurementdevice. For example the calibration source may, in use, be electricallycoupled between the live conductor on a load side of the differentialmeasurement device and the neutral conductor on a source side of thedifferential measurement device.

In use and as mentioned above current measurement apparatus comprising adifferential measurement device may provide for enhanced capabilities.With regards to fault detection such current measurement apparatus mayprovide for the combination of both absolute and differentialmeasurements to, for example, to determine if leakage is from the liveconductor or neutral conductor. Also such current measurement apparatusmay provide for ease of provision of the like of AFCI and ground faultdetection functions by relying more on measurements made by thedifferential measurement device in preference to absolute measurementswhen the currents on live and neutral are high and relying more onabsolute measurements in preference to measurements made by thedifferential measurement device when the current difference between thelive and neutral conductors is great, but by providing continuity inmeasurements by calibrating the differential response of the absolutemeasurements to the response of the differential measurement.

The present inventors have appreciated the feature of the differentialmeasurement device to be of wider applicability than hitherto described.The current measurement apparatus may therefore further comprise adifferential measurement device as described above, one of the first andsecond measurement devices being constituted by the differentialmeasurement device. The differential measurement device may therefore beoperative to sense current in a respective one of the live and neutralconductors and to sense current in the other one of the live and neutralconductors at the same time and to provide a difference output independence on the two sensed currents. The other of the first and secondmeasurement devices may be operative to measure current solely in one ofthe live and neutral conductors. This approach may allow for ameasurement device to be dispensed with to thereby save on cost andreduce complexity and size. The current measurement apparatus may beconfigured to determine a signal present in the conductor not measuredby the other of the first and second measurement devices. Morespecifically measurements made by the other of the first and secondmeasurement devices may be subtracted from measurements made by thedifferential measurement device to thereby provide measurements for eachof the live and neutral conductors. The current measurement apparatusmay therefore be configured to provide for the like or correlation andnormalisation as described elsewhere herein. The current measurementapparatus may be configured as described elsewhere herein to determinefault conditions and measure power. As described elsewhere herein acalibration signal may be applied to the live and neutral conductors ormay be applied inductively to a measurement device. Also a calibrationsignal may be applied across the live and neutral conductors or may beapplied to one of the live and neutral conductors.

According to a sixth aspect of the present invention there may beprovided a current measurement arrangement comprising plural currentmeasurement apparatus according to the fifth aspect of the presentinvention, each of the plural current measurement apparatus beingconfigured to measure current in a different one of plural liveconductors and a neutral conductor. Thus the current measurementarrangement may be operative to measure current in different phases of athree phase electrical supply.

Embodiments of the sixth aspect of the invention may comprise one ormore features of the fifth aspect of the invention. Where the currentmeasurement arrangement is configured for calibration of pluralmeasurement devices, the current measurement arrangement may comprise atleast one calibration source which is operative to apply a calibrationsignal to a measurement device in each of plural live conductors and theneutral conductor. The current measurement arrangement may comprise oneor more of the further calibration features described above.

According to a seventh aspect of the present invention there is provideda current measurement method comprising: operating each of first andsecond measurement devices to measure current in a respective one of alive conductor and a neutral conductor; and operating currentmeasurement apparatus to determine a difference between the measuredcurrents and to make a determination in dependence on the currentdifference. Embodiments of the seventh aspect of the present inventionmay comprise one or more features of the fifth or sixth aspect of thepresent invention.

According to an eighth aspect of the present invention there is providedelectrical apparatus comprising current measurement apparatus accordingto the fifth aspect of the present invention or a current measurementarrangement according to the sixth aspect, the electrical apparatusbeing configured such that the current measurement apparatus or currentmeasurement arrangement measures current passing through a part of theelectrical apparatus. Embodiments of the eighth aspect of the presentinvention may comprise one or more features of the fourth aspect of thepresent invention.

Further embodiments of the fifth to eighth aspects of the presentinvention may comprise one or more features of any other aspect of thepresent invention.

According to a ninth aspect of the present invention there is providedcurrent measurement apparatus comprising at least one measurement deviceand at least one acquisition circuit, the at least one measurementdevice being operative to measure current in at least one of a liveconductor and a neutral conductor, the at least one acquisition circuitbeing configured to have at least two different dynamic ranges and to beoperative to acquire each of plural signals from the at least onemeasurement device within a respective one of the different dynamicranges, the current measurement apparatus being operative to make pluraldifferent determinations in dependence on the plural acquired signals.

In use the at least one acquisition circuit is operative to acquireplural signals within a respective one of the different dynamic ranges.For example a first acquired signal may be in a range of 0 to 20 AmpsRMS and a second acquired signal may be in a range of 50 to 200 AmpsRMS. Furthermore the current measurement apparatus is operative to makeplural different determinations in dependence on the plural acquiredsignals. For example the current measurement apparatus may be operativeto make an arc fault determination in dependence on a signal acquiredfrom the live conductor in the 50 to 200 Amps RMS range and a currentmeasurement for metering purposes in dependence on a signal acquiredfrom the live conductor in the 0 to 20 Amps RMS range.

The current measurement apparatus may comprise a first measurementdevice operative to measure current in the live conductor, a secondmeasurement device operative to measure current in the neutralconductor, a first acquisition circuit operative to acquire pluralsignals from the first measurement device and a second acquisitioncircuit operative to acquire plural signals from the second measurementdevice. The current measurement apparatus may therefore be operative tomake at least one determination in dependence on a difference betweencurrent measured in the live and neutral conductors. For example thecurrent measurement apparatus may be operative to make an arc faultdetermination in dependence on a signal acquired from the live conductorin the 50 to 200 Amps RMS range and a ground fault determination independence on a difference between signals acquired from the live andneutral conductors in the 0 to 20 Amps RMS range. Further embodiments ofthe ninth aspect of the present invention may comprise one or morefeatures of the first aspect of the present invention.

According to a tenth aspect of the present invention there may beprovided a current measurement arrangement comprising plural currentmeasurement apparatus according to the ninth aspect of the presentinvention, each of the plural current measurement apparatus beingconfigured to measure current in a different one of plural liveconductors and a neutral conductor. Thus the current measurementarrangement may be operative to measure current in different phases of athree phase electrical supply. Embodiments of the tenth aspect of theinvention may comprise one or more features of the ninth aspect of theinvention.

According to an eleventh aspect of the present invention there isprovided a current measurement method comprising: measuring current inat least one of a live conductor and a neutral conductor with at leastone measurement device; acquiring plural signals from the at least onemeasurement device with at least one acquisition circuit, which isconfigured to have at least two different dynamic ranges, each of theplural acquired signals being within a respective one of the differentdynamic ranges; and making plural different determinations in dependenceon the plural acquired signals. Embodiments of the eleventh aspect ofthe present invention may comprise one or more features of the ninth ortenth aspect of the present invention.

According to an twelfth aspect of the present invention there isprovided electrical apparatus comprising current measurement apparatusaccording to the ninth aspect of the present invention or a currentmeasurement arrangement according to the tenth aspect of the presentinvention, the electrical apparatus being configured such that thecurrent measurement apparatus or current measurement arrangementmeasures current passing through a part of the electrical apparatus.Embodiments of the twelfth aspect of the present invention may compriseone or more features of the fourth aspect of the present invention.

Further embodiments of the ninth to twelfth aspects of the presentinvention may comprise one or more features of any other aspect of thepresent invention.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described by way of example only withreference to the following drawings, of which:

FIG. 1 is a representation of a known Ground Fault Current Interrupter(GFCI);

FIG. 2 is a representation of a known Arc Fault Current Interrupter(AFCI);

FIG. 3 is a block diagram representation of current measurementapparatus according to a first embodiment;

FIG. 4 is a block diagram representation of current measurementapparatus according to a second embodiment;

FIG. 5 is a block diagram representation of current measurementapparatus according to a third embodiment;

FIG. 6 is a block diagram representation of current measurementapparatus according to a fourth embodiment;

FIG. 7 is a block diagram representation of current measurementapparatus according to a fifth embodiment;

FIG. 8 is a block diagram representation of current measurementapparatus according to a sixth embodiment;

FIG. 9 is a block diagram representation of a three phase currentmeasurement arrangement according to the invention;

FIG. 10 is a block diagram representation of current measurementapparatus according to a seventh embodiment;

FIG. 11 is block diagram representation of current measurement apparatusaccording to an eighth embodiment;

FIG. 12 is a block diagram representation of current measurementapparatus according to a ninth embodiment;

FIG. 13 is a block diagram representation of current measurementapparatus according to a tenth embodiment;

FIG. 14 is a block diagram representation of a first embodiment ofmeasuring circuit having plural dynamic ranges;

FIG. 15 is a block diagram representation of a second embodiment ofmeasuring circuit having plural dynamic ranges;

FIG. 16 is a first example of application of the present invention;

FIG. 17 is a second example of application of the present invention;

FIG. 18 is a block diagram representation of current measurementapparatus according to an eleventh embodiment;

FIG. 19A is a first form of current measurement arrangement;

FIG. 19B is a second form of current measurement arrangement;

FIG. 19C is a third form of current measurement arrangement;

FIG. 19D is a fourth form of current measurement arrangement;

FIG. 20 is a third example of application of the present invention;

FIG. 21 is a block diagram representation of current measurementapparatus according to a twelfth embodiment; and

FIG. 22 is a block diagram representation of current measurementapparatus according to a thirteenth embodiment.

DESCRIPTION OF EMBODIMENTS

A known Ground Fault Current Interrupter (GFCI) 10 is shown in FIG. 1.The known GFCI 10 comprises a differential current transformer 12 arounda live conductor 14 and a neutral conductor 16. The neutral conductor 16is connected to ground 18. As described above the differential currenttransformer 12 is operative to measure the difference between thecurrent signals present in the live and neutral conductors 14, 16. TheGFCI 10 further comprises an amplifier and reference circuit 20, whichreceives an output from the differential current transformer 12 and isoperative to amplify the output from the differential currenttransformer 12 and compare the amplified output with a reference value,e.g. 5 mA, which is deemed a safe limit of ground current. In additionthe GFCI 10 comprises a circuit breaker comprised of a SiliconControlled Rectifier (SCR) 22, a solenoid 24, a first switch 26 inseries with the live conductor 14 and a second switch 28 in series withthe neutral conductor 16. The SCR 22 is connected in series with thesolenoid 24 and the series connected SCR 22 and solenoid 24 areconnected across the live and neutral conductors 14, 16 on the load sideof the first and second switches 26, 28. The gate of the SCR 22 isconnected to the output of the amplifier and reference circuit 20whereby a current measured by the differential current transformer 12 inexcess of the reference value is operative to switch the SCR 22 on whichin turn operates the solenoid 24. Operation of the solenoid 24 causesoperation of the first and second switches 26, 28 to thereby open thelive and neutral conductors 14, 16. Thus a measured difference incurrent signals passing through the live and neutral conductors 14, 16that exceeds a safe predetermined threshold, such as 5 mA, opens boththe live and neutral conductors 14, 16. The GFCI 10 further comprises asecond differential current transformer 30 which surrounds the live andneutral conductors 14, 16 with the coil (i.e. secondary) providing alocal current flow between the load side live and neutral for thepurpose of detecting the case were there has been an erroneous groundingof the neutral at the load side.

The operation of the GFCI will now be described with reference toFIG. 1. FIG. 1 shows a person 32 making electrical contact with the liveconductor with one part of his body whilst he is electrically connectedto ground 18 with another part of his body. The person 32 might, forexample, make electrical contact with the live conductor if the liveconductor is electrically coupled to the ungrounded casing of anelectrical appliance. At least some of the current flowing in the liveconductor follows the alternative path to ground 18 provided by the bodyof the person 32. As a result less current flows in the neutralconductor 16 than in the live conductor 14. The difference in current ismeasured by the differential current transformer 12 and if the currentdifference exceeds the predetermined safe limit, e.g. 5 mA, the GFCI isoperative as described above to open the live and neutral conductors 14,16 to thereby prevent further current from passing through the person32. If the circuit of FIG. 1 has two ground connections with the secondground connection being near the load there is the possibility ofleakage current passing through the person 32 before flowing through thesecond ground connection instead of continuing to flow through ground.Leakage current which flows through the second ground connection flowsthrough the differential current transformer 12 whereby the differentialcurrent transformer 12 fails to measure the current leakage despiteleakage current passing through the person 32. The second differentialcurrent transformer 30 near the load means that the circuit is operativeto respond to any leakage current that returns through a second groundconnection which is located at the load side.

A known Arc Fault Current Interrupter (AFCI) 40 is shown in FIG. 2. Theknown AFCI 40 comprises a differential current transformer 42 throughwhich a live conductor 44 and a neutral conductor 46 pass. The currenttransformer 42 is therefore operative to determine the differencebetween the current signals present in the live and neutral conductors44, 46. The AFCI 40 further comprises a first amplifier circuit 48,which is operative to amplify the output from the current transformer42, and logic circuitry 50, which is operative to receive the amplifiedsignal from the first amplifier 48. In addition the AFCI 40 comprises acurrent sensor 52 in series with the live conductor 44, which isoperative to measure the current signal present in the live conductor44, and a filter circuit 54, which receives an input from the currentsensor 42. The filter circuit 54 is operative to filter out normal,non-arc related signals. Thus for example the filter circuit 54comprises a band pass filter component which is operative to filter outnon-characteristic transients, which might for example be caused by loadswitching, or high frequency noise. Signals passed by the filter circuit54 are received by a second amplifier circuit 56 which passes thesignals after amplification to the logic circuitry 50. Although notshown in FIG. 2 the AFCI comprises analogue to digital convertercircuitry operative to convert the analogue signals received from thefirst and second amplifiers 48, 56 and to pass digital signals to thelogic circuitry. The logic circuitry 50 is operative in dependence onsignals received from the differential current transformer 42 and thecurrent sensor 52 to determine whether an arc is good or bad. Morespecifically the logic circuitry 50 is operative to distinguish betweennormal circuit transients, such as those caused by lamp burn out, andtransients caused by a dangerous arcing event. An output from the logiccircuitry 50 drives a circuit interrupter (not shown) of the form shownin FIG. 1 to disconnect the live and neutral conductors in the event ofdetection of a dangerous arcing event. The AFCI also comprises testcircuitry 58, which is operative on manual actuation to simulate adangerous arc and thereby trigger the circuit interrupter.

A block diagram representation of current measurement apparatus 70according to a first embodiment of the invention is shown in FIG. 3. Thecurrent measurement apparatus 70 comprises a live conductor 72 and aneutral conductor 74, which convey electrical power from a source to aload in a mains electricity circuit. The current measurement apparatusis installed, e.g. in a distribution box, in residential or businesspremises. A first shunt resistor 76 (which constitutes a firstmeasurement device) is present in series in the live conductor 72 and asecond shunt resistor 78 (which constitutes a second measurement device)is present in series in the neutral conductor 74. The currentmeasurement apparatus 70 further comprises a calibration source 80,which is electrically coupled to the live and neutral conductors 72, 74.As is described further below the calibration source 80 is operative toapply a calibration signal to the live and neutral conductors tocalibrate the first and second shunt resistors 76, 78 and theirrespective processing chains. The current measurement apparatus 70 alsocomprises a first acquisition circuit 82, which is configured to acquirean analogue current measurement from the first shunt resistor 76 andgenerate a digital representation of the analogue current measurement,and a second acquisition circuit 84, which is configured to acquire ananalogue current measurement from the second shunt resistor 78 andgenerate a digital representation of the analogue current measurement.

Each of the first and second acquisition circuits 82, 84 comprises again stage, which is operative to apply a gain to (i.e. to amplify orattenuate) the analogue current measurement before analogue to digitalconversion, and an analogue to digital converter, which is operative toperform analogue to digital conversion of the analogue currentmeasurement. The design of the first and second acquisition circuitswill be within the ordinary design capability of the person skilled inthe art other than is described herein. The outputs from the first andsecond acquisition circuits 82, 84 are received by signal processingcircuitry, which is constituted as digital signal processing circuitryor the like. The design of the first and second acquisition circuits andthe digital signal processing circuitry further to what is describedherein will be within the ordinary design capabilities of the personskilled in the art. On account of the need to maintain galvanicisolation between the live and neutral circuits the current measurementapparatus 70 comprises an isolator 86 in series between the firstacquisition circuit 82 and the signal processing circuitry. As can beseen from FIG. 3 the isolator 86 provides for communication of databetween a first power domain 96, which comprises the shunt resistor onthe live conductor and its data acquisition circuit, and a second powerdomain 98, which comprises the shunt resistor on the neutral conductorand all the remaining circuitry.

The signal processing circuitry of the current measurement apparatus 70of FIG. 3 comprises a correlation detection and correction circuit 88,which receives inputs from the first and second acquisition circuits 82,84, and first and second processing circuits 90, 92, which receiveinputs from the correlation detection and correction circuit 88. Thesignal processing circuitry also comprises non-volatile memory 94, whichis operative to store pre-stored data, such as factory calibration data,or permanently stored data, which is required to survive the apparatusbeing powered down. The signal processing circuitry further compriseslocal volatile memory, such as RAM, which is used to store data thatneed not survive power down or data of an intermediate nature, e.g. datastored for use during the course of computations.

Operation of the current measurement apparatus 70 of FIG. 3 will now bedescribed. As a first step the primary operative characteristics absentcompensation and normalisation will be described. Then calibration andnormalisation will be described. Thereafter the effect of calibrationand normalisation on normal operation of the current measurementapparatus 70 will be described.

The primary operative characteristics involve a first current signalflowing through the live conductor 72, which causes a first voltagesignal to be developed across the first shunt resistor 76. Also a secondcurrent signal flowing through the neutral conductor 74 causes a secondvoltage signal to be developed across the second shunt resistor 78. Thefirst and second acquisition circuits 82, 84 are operative to converttheir respective first and second voltage signals into correspondingfirst and second digital signals. The first and second digital signalscorrespond respectively to the first and second absolute current signalspresent in the live and neutral conductors with first and secondabsolute current signals being determinable in view of the impedance ofthe first and second shunt resistors being known, as described below.The first and second digital signals are then conveyed to the signalprocessing circuitry where compensation and normalisation, as describedbelow, are carried out. Thereafter the digital signals are then used toperform measurement and fault detection functions as described below andin particular with reference to FIGS. 10 to 13.

Calibration and normalisation will now be described. There are threemain approaches to calibration: preset, which may for example, becarried out after manufacture; by application of a calibration signalduring use; and on a self-learning basis. Each calibration approach willbe described in turn.

With regards to the preset calibration approach after manufacture thetransfer characteristics of the first and second shunt resistors 76, 78and their respective processing chains are unknown or known toinsufficient accuracy. A post manufacture calibration procedurecomprises determining the transfer characteristics with reference to acalibration standard of sufficient accuracy. The transfercharacteristics are determined over a bandwidth of operation of theshunt resistors and their processing chains. The determined transfercharacteristics are stored in non-volatile memory 94 or RAM and appliedduring normal operation as described below at the conclusion of thedescription of the different approaches to calibration.

With regards to the approach of applying a calibration signal, thecalibration source 80 is operative to apply a known calibration signalto each of the first and second shunt resistors 76, 78 during normaloperation of the current measurement apparatus 70. The calibrationsource 80 is operative at a regular predetermined intervals, e.g. oncean hour, once a day or week, sufficient to provide for maintenance ofaccuracy of measurement. Alternatively or in addition the calibrationsource 80 is operative following one or more predetermined events whenit is likely that calibration will be required, e.g. upon switch on ofthe current measurement apparatus or following detection of and responseto a fault event. The calibration source 80 is operative to apply atleast one calibration signal by one or more of the different approachesdescribed below with reference to FIGS. 4 to 8. The calibration source80 is operative is generate a calibration signal which is different fromload drawn current signals present on the live and neutral conductors72, 74 whereby the voltage signals developed across the first and secondshunt resistors 76, 78 by the calibration signal and the correspondingdigital signals are separable from the digital signals corresponding tothe load drawn current signals. The calibration signal is different fromthe load drawn current signals in respect of frequency characteristicswhereby the correlation detection and correction circuit 88 is operativeto extract the parts of the digital signals corresponding to thecalibration signal, e.g. by way of frequency analysis such as byapplication of a Fast Fourier Transform (FFT) algorithm, which isoperative to separate the parts corresponding to the calibration signaland the load drawn current signal from each other on the basis of theirdifferent frequencies. The correlation detection and correction circuit88 is operative to compare an extracted part corresponding to thecalibration signal with the known calibration signal generated by thecalibration source 80 to thereby determine the transfer characteristicsfor each of the first and second shunt resistors 76, 78 and theirrespective processing chains. The determined transfer characteristicsare stored in non-volatile memory 94 or RAM. Where preset calibration asdescribed above has already been carried out the current measurementapparatus 70 is operative to update the already stored transfercharacteristics. During the update process the factory set values areretained to thereby allow for a restore operation, to allow forcomparison with later determined values and for functions which dependon such factory set values. The stored transfer characteristics areapplied during normal operation as described below at the conclusion ofthe description of the different approaches to calibration.

The third calibration approach involves self-learning. This approachinvolves the current measurement apparatus 70 measuring the current inthe first and second shunt resistors 76, 78 over a period of time duringnormal operation and determining the transfer characteristics at leastin part for each shunt resistor and its processing chain in dependenceon these measurements. For example plural measurement are made over anextended period of time, such as seconds, minutes, hours or days, and anaverage current value determined from the plural measurements wherebythe effect of fluctuations in current caused by noise, switchingtransients and the like is reduced. The determined transfercharacteristics or partial transfer characteristics are stored innon-volatile memory 94. Alternatively already stored characteristics areupdated, e.g. in respect of an offset which has developed since initialor subsequent calibration by way of one of the other two approaches.

During normal operation of the current measurement apparatus the storedtransfer characteristics are applied by the first and second processingcircuits 90, 92 to their respective first and second digital signals.More specifically each of the first and second processing circuits 90,92 is operative to determine the absolute current flowing in itsrespective conductor 72, 74 in dependence on the transfercharacteristics of the respective shunt resistor and processing chainwhich are now accurately known following calibration as described aboveand are now reflected in the stored transfer characteristics. As willbecome apparent from the following description certain operations, suchas determining the difference in currents in the live and neutralconductors, depend on measurement of current in both live and neutralconductors 72, 74. The first and second digital signals acquired by thecurrent measurement apparatus 70 are therefore aligned with each otherto provide for accurate computations based on the first and seconddigital signals. The correlation detection and correction circuit 88 isoperative to bring the first and second digital signals into alignmentby at least one of two approaches. According to a first approach thecorrelation detection and correction circuit 88 is operative to performa cross-correlation of the first and second digital signals to determinethe phase shift which provides the greatest extent of correspondencebetween the first and second digital signals. One of the first andsecond digital signals is shifted by the determined phase shift tothereby bring the digital signals into alignment. According to a secondapproach the calibration source is operative to apply a calibrationsignal to both the first and second shunt resistors 76, 78 and a part ofthe first digital signal corresponding to the calibration signal passingthrough the first shunt resistor 76 is compared with a part of thesecond digital signal corresponding to the calibration signal passingthrough the second shunt resistor 78 to determine the phase differencebetween the first and second digital signals. One of the first andsecond digital signals is shifted by the determined phase difference tothereby bring the digital signals into alignment.

Accurate computation based on the currents measured in the live andneutral conductors requires matching of the transfer characteristics ofthe first and second shunt resistors and their processing chains inaddition to phase alignment. The current measurement apparatus 70 ofFIG. 3 is therefore operative to match the transfer characteristics ofthe first and second shunt resistors and their processing chains by oneof the two following approaches. A first graph 83 shows a first transferfunction for the first shunt resistor and its processing chain and asecond graph 85 shows a second transfer function for the second shuntresistor and its processing chain. As can be seen the first and secondtransfer functions are quite different. According to the first matchingapproach the correlation detection and correction circuit 88 isoperative to compare the first and second transfer functions asdetermined as part of the calibration process with each other and todetermine a matching function which brings the first and second transferfunctions into sufficient proximity to provide for accuracy ofcomputation based on measurement of current in both the live and neutralconductors. A matching function which is operative to bring one of thefirst and second digital signals into proximity with the other of thefirst and second digital signals is determined. A third graph 81 in FIG.3 shows the effect of a matching function which is operative to bringthe second (i.e. neutral) digital signal into proximity with the first(i.e. live) digital signal. According to the second approach thecorrelation detection and correction circuit 88 is operative todetermine first and second matching functions, with the first matchingfunction being operative to bring the first digital signal intoproximity with an object function and the second matching function beingoperative to bring the second digital signal into proximity with theobject function. The object function is of a form which provides forease of execution of subsequent computations. For example the objectfunction is a linear function. A fourth graph 95 in FIG. 3 shows theeffect of first and second matching functions which are operative tobring their respective first (i.e. live) and second (i.e. neutral)digital signals into proximity with a linear function. According to thefirst approach the matching function characteristics are stored innon-volatile memory 94 or RAM for application by one of the first andsecond processing circuits 90, 92 to its respective digital signal.According to the second approach the first and second matching functioncharacteristics are stored in non-volatile memory 94 or RAM with thefirst processing circuit 90 being operative to apply the first matchingfunction to the first digital signal and the second processing circuit90 being operative to apply the second matching function to the seconddigital signal.

Current measurement apparatus 100 according to a second embodiment isshown in FIG. 4. Components in common with the embodiment of FIG. 3 aredesignated by like reference numerals and the reader's attention isdirected to the description provided above with reference to FIG. 3 fora description of such common components. Components particular to theembodiment of FIG. 4 will now be described. The calibration source 80comprises a calibration resistor 102 in series with a switch 104 withthe series arrangement of calibration resistor 102 switch 104 beingelectrically connected between the live and neutral conductors 72, 74.The current measurement apparatus 100 of FIG. 4 further comprises acalibration control unit 106, which forms part of the signal processingcircuitry described above. The calibration control unit 106 is operativeto turn the switch 104 on and off in a predetermined fashion withcurrent passing between the live and neutral conductors when the switch104 is closed. Turning the switch on and off in this fashion thereforemodulates the current signals present on the live and neutral conductorswith the calibration control unit 106 operating the switch so as toimpress a characteristic current signal on both the live and neutralconductors. The characteristic current signal is measured by each of thefirst and second shunt resistors 76, 78 and extracted by the correlationdetection and correction circuit 88 as described above with reference toFIG. 3 to thereby provide for calibration of the first and second shuntresistors and their processing chains.

Current measurement apparatus 110 according to a third embodiment isshown in FIG. 5. Components in common with the embodiments of FIGS. 3and 4 are designated by like reference numerals and the reader'sattention is directed to the description provided above with referenceto FIGS. 3 and 4 for a description of such common components. Componentsparticular to the embodiment of FIG. 5 will now be described. Instead ofthe first shunt resistor 76 of FIGS. 3 and 4 the embodiment of FIG. 5comprises a current transformer 112 which is configured as describedelsewhere herein such that it is operative to measure current flowing inthe live conductor 72. Although not shown in FIG. 5 the currenttransformer comprises a burden resistor connected across the coil of thetransformer and which is operative in accordance with normal designpractice. In view of the inherently isolating characteristic of thecurrent transformer 112 there is no need to provide for isolationbetween the processing chains of the current transformer 112 and thesecond shunt resistor 78. A power domain, which is indicated by box 98in FIG. 5 and which is isolated from the live conductor, comprises theshunt resistor on the neutral conductor and all the data processingcircuitry. The embodiment of FIG. 5 is operative as described above withfollowing exception. The current in the live conductor 72 induces acorresponding current in the current transformer which is then subjectto acquisition and processing to provide a first digital signal asdescribed above. The calibration source 80 is operative as describedabove with reference to FIG. 4 to impress a characteristic currentsignal on the live and neutral conductors with the characteristiccurrent signal on the live conductor inducing a corresponding inducedcharacteristic current signal in the current transformer. The part ofthe first digital signal corresponding to the induced characteristiccurrent signal is extracted by the correlation detection and correctioncircuit 88 as described above to thereby provide for calibration of thecurrent transformer 112 and its processing chain.

Current measurement apparatus 120 according to a fourth embodiment isshown in FIG. 6. Components in common with the embodiments of FIGS. 3 to5 are designated by like reference numerals and the reader's attentionis directed to the description provided above with reference to FIGS. 3to 5 for a description of such common components. Components particularto the embodiment of FIG. 6 will now be described. The currentmeasurement apparatus 120 comprises a calibration conductor 122, whichpasses through the current transformer 112. The distal end of thecalibration conductor 122 is electrically connected to the neutralconductor 74 on the load side of the second shunt resistor 78. Theproximal end of the calibration conductor 122 is electrically connectedto a signal output of another form of calibration source 124. A lowvoltage line of the calibration source 124 is electrically connected onthe source side of the second shunt resistor 78. Therefore signalsgenerated by this form of calibration source 124 pass out though thecalibration conductor 122 and return to the calibration source 124 byway of the neutral conductor 74 and second shunt resistor 78. Thisarrangement replaces the calibration source 80 of FIGS. 3 to 5. A powerdomain, which is indicated by box 98 in FIG. 6 and which is isolatedfrom the live conductor, comprises the shunt resistor on the neutralconductor and all the data processing circuitry. The calibration source124 of FIG. 6 is operative to generate a characteristic calibrationsignal which passes through the calibration conductor 122 and therebyinduces a corresponding signal in the current transformer 112 beforepassing through the second shunt resistor 78 wherein a correspondingvoltage signals developed. The correlation detection and correctioncircuit 88 is operative to extract the parts of the first and seconddigital signals corresponding to the calibration signal to therebyprovide for calibration of the current transformer 112 and itsprocessing chain and the second shunt resistor 78 and its processingchain as described above.

Current measurement apparatus 130 according to a fifth embodiment isshown in FIG. 7. Components in common with the embodiments of FIGS. 3 to6 are designated by like reference numerals and the reader's attentionis directed to the description provided above with reference to FIGS. 3to 6 for a description of such common components. Components particularto the embodiment of FIG. 7 will now be described. The currentmeasurement apparatus 130 of FIG. 7 comprises a first currenttransformer 112 instead of the first shunt resistor 76 of FIG. 3 and asecond current transformer 132 instead of the second shunt resistor 78of FIG. 3. In addition the current measurement apparatus 130 of FIG. 7comprises a calibration conductor 134 which is electrically connected ata first end to a signal output from the calibration source 124, passesthrough the first and second current transformers 112, 132 in turnbefore being electrically connected to the return signal terminal of thecalibration source 124. A power domain, which is indicated by box 136 inFIG. 7 and which is isolated from the live and neutral conductors,comprises all the data processing circuitry. The calibration source 124is operative as described above with reference to FIG. 6 to generate acalibration signal. The calibration signal passes through thecalibration conductor 134 and thereby induces a corresponding signal ineach of the first and second current transformers 112, 132. Thecorrelation detection and correction circuit 88 is operative to extractthe parts of the first and second digital signals corresponding to thecalibration signal to thereby provide for calibration of the first andsecond current transformers 112, 132 and their respective processingchains as described above.

Current measurement apparatus 140 according to a sixth embodiment isshown in FIG. 8. Components in common with the embodiments of FIGS. 3 to7 are designated by like reference numerals and the reader's attentionis directed to the description provided above with reference to FIGS. 3to 7 for a description of such common components. Components particularto the embodiment of FIG. 8 will now be described. As with theembodiment of FIG. 7 the embodiment of FIG. 8 comprises first and secondcurrent transformers 112, 132 which are operative to measure current inthe live conductor 72 and the neutral conductor 74 respectively. Thecurrent measurement apparatus 140 of FIG. 8 further comprises firstcalibration conductor 142, a second calibration conductor 144 and aswitch/multiplexer circuit 146. An output from the calibration source124 is received by the switch/multiplexer circuit 146. A first outputfrom the switch/splitter circuit 146 is electrically coupled to thefirst calibration conductor 142 and a second output from theswitch/splitter circuit 146 is electrically coupled to the secondcalibration conductor 144. The first calibration conductor 142 passesthrough the first current transformer 112 before being electricallyconnected to the neutral conductor 74. The second calibration conductor144 passes through the second current transformer 132 before beingelectrically connected to the neutral conductor 74. A first calibrationsignal passing through the first current transformer 112 by way of thefirst calibration conductor 142 therefore induces a corresponding signalin the first current transformer 112 and a second calibration signalpassing through the second current transformer 132 by way of the secondcalibration conductor 144 therefore induces a corresponding signal inthe second current transformer 132. The correlation detection andcorrection circuit 88 is operative as described above to extract theparts of the first and second digital signals corresponding to the firstand second calibration signals and to thereby determine the transfercharacteristics of each of the first and second current transformers112, 132 and their respective processing chains. The calibrationcharacteristics are then determined and stored as described above. Thecalibration source 124 and the switch/splitter circuit 146 are operativeto provide for different approaches to calibration signal generation asfollows. According to a first approach the switch/splitter circuit 146is operative to apply the same form of calibration signal to each of thefirst and second calibration conductors 142, 144 at the same time.According to a second approach the switch/splitter circuit 146 isoperative to apply the same form of calibration signal to each of thefirst and second calibration conductors 142, 144 in turn. According to athird approach the switch/splitter circuit 146 is operative to apply adifferent calibration signal to the first and second calibrationconductors 142, 144 either at the same time or in turn. Application ofdifferent calibration signals may be appropriate where the currenttransformers have different characteristics, e.g. where the firstcurrent transformer is configured for measurement of large amplitudesignals and the second current transformer is configured for measurementof small amplitude signals.

A three phase current measurement arrangement 150 according to theinvention is shown in FIG. 9. The three phase current measurementarrangement 150 comprises first to third live conductors 152, 154, 156and a neutral conductor 158 through which electrical power is drawn by aload from a source. First to third shunt resistors 160, 162, 164 areprovided in series with a respective one of the first to third liveconductors 152, 154, 156 and a fourth shunt resistor 166 is provided inseries with the neutral conductor 158. A first calibration source 168 isconfigured to apply a calibration signal to the first live conductor 152and the neutral conductor 158. A second calibration source 170 isconfigured to apply a calibration signal to the second live conductor154 and the neutral conductor 158. A third calibration source 172 isconfigured to apply a calibration signal to the third live conductor 156and the neutral conductor 158. In alternative forms of the currentmeasurement arrangement 150 of FIG. 9 one or more of the shunt resistorsmay be replaced with a current transformer. Therefore each of the firstto third calibration sources 168, 170, 172 is operative to applycalibration signals according to one or more of the approaches describedabove with reference to FIGS. 4 to 8. The current measurementarrangement 150 of FIG. 9 further comprises first to fourth acquisitioncircuits 174, 176, 178, 180 which are operative to acquire signalsmeasured by a respective one of the first to fourth shunt resistors 160,162, 164, 166. Each of the first to fourth acquisition circuits 174,176, 178, 180 comprises a gain stage and is operative as describedabove. The current measurement arrangement 150 of FIG. 9 also comprisesfirst to third isolation circuits 182, 184, 186 in series with arespective one of the first to third acquisition circuits 174, 176, 178and thereby operative to maintain galvanic isolation between and amongstthe live conductors and neutral conductor. The current measurementarrangement 150 of FIG. 9 further comprises: a correlation detection andcorrection circuit 188, which receives an input from each of the firstto fourth acquisition circuits 174, 176, 178, 180; and first to fourthprocessing circuits 190, 192, 194, 198 which each receive an input fromthe correlation detection and correction circuit 188 and a respectiveinput from the first to fourth acquisition circuits 174, 176, 178, 180.The correlation detection and correction circuit 188 has a control input189 which provides for input of control data and deterministiccalibration signal data. In addition the current measurement arrangement150 of FIG. 9 comprises non-volatile memory 200 which is operative tostore data used in calibration and in other signal processingoperations. The correlation detection and correction circuit 188, thefirst to fourth processing circuits 190, 192, 194, 198 and thenon-volatile memory 200 are operative to provide for calibration,alignment and normalisation of measurements made by the first to fourthshunt resistors 160, 162, 164, 166 as described above with reference toFIG. 3. Digital signals generated by the first to fourth processingcircuits 190, 192, 194, 198 are used in subsequent computations andoperations as described below with reference to FIGS. 10 to 13.

Current measurement apparatus 220 according to a seventh embodiment isshown in FIG. 10. Components in common with the embodiment of FIG. 4 aredesignated by like reference numerals and the reader's attention isdirected to the description provided above with reference to FIG. 4 fora description of such common components. Although the embodiment of FIG.10 comprises the two shunt resistors and the calibration sourcearrangement of the embodiment of FIG. 4 the arrangements of currentmeasurement device and calibration source comprised in the embodimentsof FIGS. 5 to 8 may be used instead. Components particular to theembodiment of FIG. 10 will now be described. The current measurementapparatus 220 of FIG. 10 comprises a differencing circuit 222, whichreceives an input from each of the first and second processing circuits90, 92, a ground fault response filter 224, which receives an input fromthe differencing circuit 222, and a ground fault determination circuit226, which receives an input from the ground fault response filter 224.The current measurement apparatus 220 of FIG. 10 also comprises a firstarc waveform detector 228, which receives an input from the secondprocessing circuit 92, an arc fault filter circuit 230, which receivesan input from the first arc waveform detector 228, and an arc faultdetermination circuit 232, which receives an input from the arc faultfilter circuit 230. The current measurement apparatus 220 of FIG. 10further comprises a second arc waveform detector 234, which receives aninput from the first processing circuit 90, with an output from thesecond arc waveform detector 234 being received for processing by thearc fault filter circuit 230 and for further processing thereafter bythe arc fault determination circuit 232. In addition current measurementapparatus 220 of FIG. 10 comprises an event categoriser and generatorcircuit 236, which receives an input from each of the ground faultdetermination circuit 226 and the arc fault determination circuit 232and also from the second acquisition circuit 84. Absolute measurementsreceived from the second acquisition circuit 84 are used by the eventcategoriser and generator circuit 236 to determine whether or not aresponse should be generated or to determine if measurements and derivedmeasurements should be stored or subject to analysis. The eventcategoriser and generator circuit 236 comprises reset 237 andconfiguration control inputs 238. Components particular to FIG. 10 arecomprised in digital signal processing circuitry. Furthermore the eventcategoriser and generator circuit 236 generates output signals 240 forcontrol of circuit breakers, communications circuitry, data storage anda display unit (not shown).

Operation of the current measurement apparatus 220 of FIG. 10 will nowbe described. The differencing circuit 222 is operative on the first andsecond digital signals received from the first and second processingcircuits 90, 92 to determine the difference between the two receivedsignals and to generate a digital difference signal, which correspondsto the difference between the absolute current signals in the live andneutral conductors 72, 74. As described above the difference between thecurrent signals in the live and neutral conductors is indicative ofcurrent leakage as may be caused by a ground fault. The ground faultresponse filter 224 comprises a band pass filter, which is operative onthe received digital difference signal to filter out: higher frequencysignals, such as arc signals and other signals characteristic of normalcircuit operation, such as load switching, or high frequency noise; andlow frequency noise and any dc offset, which might be present. Theground fault determination circuit 226 is operative to compare thefiltered digital difference signal with a predetermined thresholdleakage value, such as 30 mA, and to generate an output if the filtereddigital difference signal exceeds the threshold leakage value for apredetermined period of time, such as 100 mS. In another form the groundfault determination circuit 226 is operative to compare the amplitudeover time with a function which lies within a safe limit of leakagecurrent as determined by a regulatory body, such as the NationalElectrical Manufacturers Association (NEMA). For example the functionmay lie within the maximum non-linear current versus time curvespecified by UL for Class A GFCIs but not set such a low limit ofleakage current as typical Class A GFCIs to thereby reduce thelikelihood of false or unwarranted ground fault detection.

The first arc waveform detector 228 comprises a band pass filter whichis operative on the second digital signal to filter outnon-characteristic high frequency transients, which might for example becaused by load switching, or high frequency noise and low frequencysignals, which might for example be caused by leakage current or normalcircuit operation, such as mains frequency components. The first arcwaveform detector 228 is also operative to analyse the second digitalsignal to identify candidate waveform profiles which might be indicativeof an arcing condition. More specifically the first arc waveformdetector 228 looks for characteristic waveform profiles in the seconddigital signal on an ongoing basis and saves portions of the seconddigital signal which meet the analytical criteria. The candidatewaveform profiles are conveyed to the arc fault filter circuit 230,which is operative to compare each received candidate waveform profilewith a library of waveform profiles, which are characteristic of arcingbehaviour and of non-arcing behaviour. Candidate waveform profiles whichare determined to be indicative of arcing behaviour are conveyed to thearc fault determination circuit 232. The arc fault determination circuit232 is operative on each received candidate waveform profile to comparethe peak RMS current of the waveform with a threshold series arc value,such as 5 Amps. If the peak RMS current exceeds the threshold series arcvalue the arc fault determination circuit 232 is operative to generate aseries arc fault detection output. A series arc fault detection outputis indicative of a series arc fault between live and ground.

The second arc waveform detector 234 comprises a band pass filter whichis operative on the first digital signal to filter outnon-characteristic high frequency transients in the same fashion as thefirst arc waveform detector 228. The second arc waveform detector 234 isalso operative in the same fashion as the first arc waveform detector228 to analyse the first digital signal to identify candidate waveformprofiles which might be indicative of an arcing condition. The candidatewaveform profiles are conveyed to the arc fault filter circuit 230,which is operative as described above. The arc fault determinationcircuit 232 is operative on candidate waveform profiles received fromthe arc fault filter circuit 230 to compare the peak RMS current of theeach waveform with a threshold series arc value, such as 5 Amps. Asdescribed above if the peak RMS current exceeds the threshold series arcvalue the arc fault determination circuit 232 is operative to generate aseries arc fault detection output. A series arc fault detection outputgenerated in dependence on a first digital signal from the firstprocessing circuit 90 is indicative of a series arc fault betweenneutral and ground.

The arc fault filter circuit 230 is also operative to detect parallelarc faults. More specifically a first candidate waveform profile, whichis received from the first processing circuit 90, and a second candidatewaveform profile, which is received from the second processing circuit92, which are of corresponding shape and which occur within apredetermined time of each other are identified by the arc fault filtercircuit 230 as being indicative of a parallel arc fault between the liveand neutral conductors 72, 74. The predetermined time of occurrence ofthe first and second candidate waveform profiles is set to take accountof circuit conditions, i.e. a likely time for arcing event on the liveconductor to propagate to the neutral conductor. One of the first andsecond candidate waveforms is conveyed to the arc fault determinationcircuit 232 along with data identifying the candidate waveform as beingindicative of a possible parallel arc fault. The arc fault determinationcircuit 232 is operative to

compare the peak RMS current of the received candidate waveform with athreshold parallel arc value, such as 75 Amps. If the peak RMS currentexceeds the threshold parallel arc value the arc fault determinationcircuit 232 is operative to generate a parallel arc fault detectionoutput. A parallel arc fault detection output is indicative of aparallel arc fault between live and neutral.

The event categoriser and generator circuit 236 receives the datasignals described above which indicate the occurrence of a ground fault,a series arc fault on the live conductor, a series arc fault on theneutral conductor and a parallel arc fault between the live and neutralconductors. The event categoriser and generator circuit 236 is operativeto respond in one or more different fashions in dependence on receipt ofthe data signals. One response involves the event categoriser andgenerator circuit 236 generating a circuit breaker control signal whichis operative to actuate a circuit breaker to break the live and neutralconductors and thereby stop the fault condition. Another responseinvolves the event categoriser and generator circuit 236 controlling alocal display device to indicate a status change of the currentmeasurement apparatus 220, such as the detection of one or more faults,operation of a circuit breaker, operation of a reset procedure and thelike. A further response involves the event categoriser and generatorcircuit 236 conveying data to a remote location, such as a dataprocessing centre, by way of wired or wireless communications circuitry.A yet further response involves the event categoriser and generatorcircuit 236 storing data in local data storage, e.g. for later analysisand comparison with later determined data or later communication to aremote location. The event categoriser and generator circuit 236 is alsoconfigured to receive an input from the second acquisition circuit 84,which is operative to provide a digital signal corresponding to thecurrent signal present on the neutral conductor. The input from thesecond acquisition circuit 84 provides the event categoriser andgenerator circuit 236 with absolute current data representing theabsolute level of current flowing in the neutral conductor. The eventcategoriser and generator circuit 236 is operative to analyse theabsolute current data and to determine circuit conditions and makedecisions as to how to respond to data signals received from the groundfault determination circuit 226 and the arc fault determination circuit232. For example analysis of the absolute current data may determinethat the loading conditions have changed, e.g. on account of addition ofa new load or change in operation of an existing load, which causes apeak in current drawn by the load. The event categoriser and generatorcircuit 236 is operative in dependence on such a determination tore-categorise a fault condition, such as a series arc fault on theneutral conductor, as non-dangerous with no action or delayed actionbeing taken by the event categoriser and generator circuit 236, e.g. inrespect of operation of a circuit breaker. Operation of the reset inputof the event categoriser and generator circuit 236 causes the eventcategoriser and generator circuit 236 to carry out a reset procedure.The reset input may be operated manually, e.g. by pressing of a resetbutton, or remotely, e.g. by sending of a reset signal from a controlcentre. The configuration control input of the event categoriser andgenerator circuit 236 is used for one or more of various purposesincluding the configuration of the current measurement apparatus 220 tocarry out a subset of the above described fault condition detectionoperations, changing the operation of the current measurement apparatus,e.g. in respect of the threshold values used during fault detection,firmware updates and the like. Certain of such purposes are describedfurther below.

An eighth embodiment of current measurement apparatus 250 is shown inFIG. 11. Components in common with the embodiments of FIGS. 5 and 10 aredesignated by like reference numerals and the reader's attention isdirected to the description provided above with reference to FIGS. 5 and10 for a description of such common components. Components particular tothe embodiment of FIG. 11 will now be described. The current transformer112 and the shunt resistor 78 of the embodiment of FIG. 4 are exchanged.The current measurement apparatus 250 comprises a potential divider 252comprising series connected resistors connected between the live andneutral conductors 72, 74, which provide an attenuated voltage signalwhich corresponds to the voltage signal between the live and neutralconductors 72, 74. The current measurement apparatus 250 furthercomprises a third acquisition circuit 254, which comprises a gain stageand an analogue to digital converter, which is operative to generate athird digital signal that corresponds to the voltage signal between thelive and neutral conductors. The current measurement apparatus 250 alsocomprises a power measurement circuit 256, which receives the thirddigital signal and the first digital signal. The power measurementcircuit 256 therefore receives a digital signal corresponding to thevoltage signal between the live and neutral conductors and the currentsignal in the live conductor. The power measurement circuit 256 isoperative to determine power consumption on the basis of the first andthird digital signals. The power measurement circuit 256 is thereafteroperative in one or more fashions. According to one approach the powermeasurement circuit 256 is operative to display the determined powerconsumption data on a display unit. According to another approach thepower measurement circuit 256 is operative by way of the communicationscircuitry to convey the determined power consumption data to a to remotelocation, such as a control centre, by wireless or wired means.According to a further approach the power measurement circuit 256 isoperative to store the determined power consumption data in local datastorage, e.g. for later analysis and comparison with later determineddata or later communication to a remote location.

The event categoriser and generator circuit 236 of the embodiment ofFIG. 11 is configured to perform functions further to those describedabove with reference to the embodiment of FIG. 10. There are threedifferent categories of function, namely parameter change,programmability and learning. Before describing such further functionsthe configuration of the current measurement apparatus 250 of FIG. 11will now be considered further. As can be seen from FIG. 11 the currentmeasurement apparatus 250 is configured for ground fault detection butlacks the capability to perform series and parallel arc fault detection.Therefore the current measurement apparatus 250 comprises a differencingcircuit 222, which receives an input from each of the first and secondprocessing circuits 90, 92, a ground fault response filter 224, whichreceives an input from the differencing circuit 222, and a ground faultdetermination circuit 226, which receives an input from the ground faultresponse filter 224. The operation of the differencing circuit 222, theground fault response filter 224 and the ground fault determinationcircuit 226 are as described above with reference to FIG. 10 with thesecomponents being further configured and operable as follows.

The first category of function is parameter change. This category offunction involves changing parameters used by the current measurementapparatus 250 in detecting particular fault conditions and makingparticular measurements. More specifically one or more of the followingparameters are changeable. With regards to the differencing circuit 222an accuracy to which the difference between the first and second digitalsignals is determined is changeable and a frequency of determination ofthe difference is changeable. With regards to the ground fault responsefilter 224 the cut off frequency of the low pass filter is changeable.With regards to the ground fault determination circuit 226 thepredetermined threshold leakage value and the predetermined period oftime are changeable. For example where the ground fault determinationcircuit 226 is configured to perform a Class A GFCI function with apredetermined threshold leakage value of 4 to 6 mA RMS the predeterminedthreshold leakage value may be changed to 20 mA RMS to perform an RCDfunction that meets European regulations. Where the current measurementapparatus is configured for over current detection as described belowwith reference to FIG. 12 an over current threshold value is changeable,e.g. amongst 5 Amps RMS, 15 Amps RMS and 30 Amps RMS. Similarlyparameters used in series and parallel arc fault detection arechangeable.

The second category of function is programmability. This category offunction involves the configuration of the current measurement apparatus250 being changed to effect different combinations of measurements andfault condition detection operations or to effect a change in a faultcondition detection or measurement procedure. A change of configurationis effected by way of the configuration control input to the eventcategoriser and generator circuit 236. In one form the configurationcontrol input is constituted in a form suitable for manual change, e.g.the configuration control input may be in the form of DIP switches. Inanother form the configuration control input is constituted as acommunications port to which a local Personal Computer (PC) or the likeis connected with the PC being operative to change the configuration byway of the communications port. In yet another form the configurationcontrol input is constituted as a communication link to a remotelocation, such as a control centre, which is operative to change theconfiguration by way of the communication link. The configuration ischangeable at deployment of the current measurement apparatus toconfigure the current measurement apparatus for a particularapplication. The configuration is also changeable after deployment, e.g.locally by way of the communications port or remotely by way of thecommunications link, to take account of changing usage requirements or achange in regulatory requirements. The configuration is changeable inone or more of the following fashions. Although not shown in FIG. 11 thecurrent measurement apparatus comprises when in a different form the arcfault detection capabilities of the embodiment of FIG. 10 and also anover current detection capability as described below with reference toFIG. 12. The current measurement apparatus 250 is configured to enabledifferent combinations of function. For example and according to a firstconfiguration the current measurement apparatus is operative to measurecurrent for metering purposes and to detect ground faults. According toa second example configuration the current measurement apparatus isoperative to measure current for metering purposes and to detect seriesarc faults from live and neutral. According to a third exampleconfiguration the current measurement apparatus is operative to measurecurrent for metering purposes, to detect ground faults and to detectparallel arc faults. In addition the configuration is changeable withregards to how the current measurement apparatus is operative to carryout a particular function. For example a process for determining a faultcondition is changed to take account of a hitherto unused measurement,such as fresh use of a voltage signal in parallel arc fault detection.

The third category of function is learning. This category of functioninvolves the current measurement apparatus 250 changing itsconfiguration, changing how a particular function is performed orchanging a parameter used in a function, with a change being effected independence on measurements made or fault conditions detected by thecurrent measurement apparatus 250. Thus the current measurementapparatus is operative to change its configuration of itself and withoutinstigation from an outside agent. For example if the currentmeasurement apparatus is operative to determine that a particular arcevent is detected only when a new load is connected to the live andneutral conductors the current measurement apparatus adapts itscategorisation process to categorise the particular arc event asnon-dangerous. Alternatively the current measurement apparatus changesthe threshold arc value parameter to make the current measurementapparatus less liable to detect the particular arc event.

A ninth embodiment of current measurement apparatus 260 is shown in FIG.12. Components in common with the embodiment of FIGS. 10 and 11 aredesignated by like reference numerals and the reader's attention isdirected to the description provided above with reference to FIGS. 10and 11 for a description of such common components. Componentsparticular to the embodiment of FIG. 12 will now be described. Thecurrent measurement apparatus 260 comprises an over current filter 262,which receives an input from the first acquisition circuit 82, and anover current detector 264, which receives an input from the over currentfilter 262. The over current detector 264 generates an output which isreceived by the event categoriser and generator circuit 236. The currentmeasurement apparatus 260 also comprises a second arc fault filtercircuit 266, which receives an input from the second arc waveformdetector 234, and a second arc fault determination circuit 268, whichreceives an input from the second arc fault filter circuit 266. Incommon with the embodiment of FIG. 10 the embodiment of FIG. 12 isoperative to detect series arc faults from each of live and neutral.However the embodiment of FIG. 12 provides for different forms ofparallel arc fault detection. A series live arc is detected by the firstarc waveform detector 228, the first arc fault filter circuit 230, andthe first arc fault determination circuit 232. A series neutral arc isdetected by the second arc waveform detector 234, the second arc faultfilter circuit 266 and the second arc fault determination circuit 268.In the embodiment of FIG. 10 a single arc fault filter circuit 230 and asingle arc fault determination circuit 232 are operative to detectseries and parallel arcing events.

According to one form of parallel arc fault detection the amplitude andtiming of the waveforms measured in the current transformer 112 and theshunt resistor 76 are analysed to detect an arcing event whichprogresses through one of the live and neutral conductors and returnsthrough the other of the live and neutral conductors. According toanother form of parallel arc fault detection the characteristics of thefirst and second arc fault filter circuits 230, 266 are changed, e.g. inrespect of their threshold values, to take account of the increasedlevels of peak current seen in parallel arcs compared with series arcs.In addition the voltage signal measured between the live and neutralconductors is analysed with a peak present in the voltage signalwaveform being indicative of an arcing event.

The over current filter 262 of FIG. 12 is operative to receive a digitalsignal from the first acquisition circuit 82, the digital signalcorresponding to the current signal flowing through the live conductor.The over current filter 262 comprises a band pass filter which isoperative to filter low frequency signals, such as a mains frequencycomponent, and high frequency signals, such as transients and noise. Theover current detector 264 receives the filtered digital signal andcompares the received digital signal with an over current thresholdvalue, such as 15 Amps RMS, If the received digital signal exceeds theover current threshold value an over current detect data is generatedand conveyed to the event categoriser and generator circuit 236. Theevent categoriser and generator circuit 236 is operative in dependenceon the over current detect data to one or more of: operate a circuitbreaker, provide an indication on a display device, convey the data byway of a communications link and store the data in local data storage.

Further to the functions described above the event categoriser andgenerator circuit 236 of the embodiment of FIG. 12 is operative asfollows. The event categoriser and generator circuit 236 is operative toupon receipt of plural fault condition data to make a decision as to howto respond based on the types of fault detected. For example if groundand arc faults are detected the event categoriser and generator circuit236 is operative to disregard the arc fault and operate a circuitbreaker in accordance with ground fault requirements in respect ofpromptness of response. Alternatively if a series arc fault and aparallel arc fault are detected the event categoriser and generatorcircuit 236 is operative to operate a circuit breaker in dependence onthe series arc fault, to store data relating to both faults and toreport both arc faults to the remote location. The event categoriser andgenerator circuit 236 is also operative to provide for interactionbetween and amongst outputs from the power measurement circuit 256, thecurrent detector 264 and the fault detector circuits. For example if thepower measurement circuit 256 is operative to measure a sudden increasein power consumption a parallel arc fault may be disregarded if the arcis of no undue magnitude. Alternatively if an over current event and aseries arc fault are detected precedence is given to the over currentevent with regards to how the event categoriser and generator circuit236 responds by operation of a circuit breaker and indication andreporting of the faults.

A tenth embodiment of current measurement apparatus 280 is shown in FIG.13. Components in common with the embodiment of FIGS. 10 and 11 aredesignated by like reference numerals and the reader's attention isdirected to the description provided above with reference to FIGS. 10and 11 for a description of such common components. Componentsparticular to the embodiment of FIG. 13 will now be described. Insteadof the first data acquisition circuit 82 of FIGS. 10 and 11 theembodiment of FIG. 13 comprises a first data acquisition circuit 282which is operative to generate one digital signal which is provided tothe correlation detection and correction circuit 88 as describedelsewhere and another digital signal which is provided to the first arcwaveform detector 228. Hence the arc detection circuitry of theembodiment of FIG. 13 is operative on digitals signals received directlyfrom the first data acquisition circuit 282 instead of from the firstprocessing circuit 90. Otherwise the operation of the arc detectioncircuitry and the current measurement apparatus 280 is as describedabove.

The embodiments of FIGS. 10 to 13 can be applied in a three phasearrangement. More specifically the outputs from the measurement deviceswhich are operative to measure the current in the three live conductorsand the neutral conductor are aggregated to determine the differencebetween the sum of the currents flowing in the live conductors and thecurrent returning through the neutral conductor. Three voltage measuringpotential dividers are operative to measure the voltage signal presentbetween a respective one of the three live conductors and the neutralconductor. Faults on each phase can thereby be determined.

A measuring circuit 290 having plural dynamic ranges according to afirst embodiment is shown in FIG. 14. The measuring circuit 290comprises a live conductor 292 in which a shunt resistor 294 isconnected in series. A first gain stage 296 receives an output from theshunt resistor 294. A first analogue to digital converter 298 receivesan output from the first gain stage 296. A second gain stage 300receives an output from the shunt resistor 294. A second analogue todigital converter 302 receives an output from the second gain stage 300.The first measurement chain comprising the first gain stage and analogueto digital converter 296, 298 are configured for large signalmeasurement, e.g. for the purpose of arc fault detection. The secondmeasurement chain comprising the second gain stage and analogue todigital converter 300, 302 are configured for small signal measurement,e.g. for the purposes of ground fault detection and current measurementfor metering purposes. Therefore the two measurement chains areconfigured to be operative in different dynamic ranges. Morespecifically the first gain stage 296 and the first analogue to digitalconverter 298 are configured to be operative in a small dynamic range,such as 0 to 5 Amps RMS and the second gain stage 300 and the secondanalogue to digital converter 302 are configured to be operative in alarge dynamic range, such as 0 to 100 Amps RMS. Furthermore the firstand second analogue to digital converters 296, 302 are clocked atdifferent frequencies. More specifically the first analogue to digitalconverter 296 is operative at a high frequency sufficient to acquire afast arcing event and the second analogue to digital converter 302 isoperative at a low frequency sufficient to acquire slower ground faultevents. The measuring circuit 290 of FIG. 14 is applied in the currentmeasurement apparatus described above with reference to FIGS. 3 to 13.

A measuring circuit 310 having plural dynamic ranges according to asecond embodiment is shown in FIG. 15. The measuring circuit 310comprises a live conductor 312 in which a shunt resistor 314 isconnected in series. An adjustable gain stage 316 receives an outputfrom the shunt resistor 314. An analogue to digital converter 318receives an output from the adjustable gain stage 316. A range detectorcircuit 320 receives an output from the analogue to digital converter318 and provides an input to a gain selector 322. A data output from theanalogue to digital converter 318 and a gain value output from the gainselector 322 are used to determine a compromise with regards to measuredvalues between resolution and dynamic range. The measuring circuit 310is operative as follows. During measurement of voltage signals developedacross the shunt resistor 314 the range detector 320 is operative todetermine which one of plural ranges the digital signal output from theanalogue to digital converter 318 falls within, e.g. within a 0 to 5 AmpRMS range or a 0 to 100 Amp RMS range. The gain selector 322 isoperative in dependence on the range determination to select a gain forthe adjustable gain stage 316. If the range determination registers nochange in range there is no change to the gain of the adjustable gainstage 316. If the range determination registers an increase or decreasein range the gain selector is operative to respectively reduce orincrease the gain of the adjustable gain stage 316. Thus the measuringcircuit 310 of FIG. 15 is operative to accommodate changes in dynamicrange of a current signal flowing through the live conductor 312. Thetwo graphs in FIG. 15 show the change in gain in response to changes inthe amplitude of the measured signal. The measuring circuit 310 of FIG.15 is applied in the current measurement apparatus described above withreference to FIGS. 3 to 13. More specifically outputs 324 from theanalogue to digital converter 318 and gain selector 322 are provided tothe processing circuitry of the current measurement apparatus forcontrol of dynamic range versus precision.

A first example of application of the present invention is shown in FIG.16. The apparatus 330 of FIG. 16 is of a form appropriate forapplication in a single consumer environment to augment the capabilitiesof an electricity meter, such as a home or business premises to includefault detection within the premises. Typically the apparatus 330 isinstalled at or near the point of entry of live and neutral mainselectricity conductors to the supply location. The apparatus may also bein the distribution box to perform point of branch sub metering. Theapparatus 330 of FIG. 16 comprises electricity supply conductors 332,current measurement apparatus 334 and voltage measurement apparatus 336.The current measurement apparatus 334 and voltage measurement apparatus336 are as described above with reference to FIGS. 3 to 13. Theapparatus 330 of FIG. 16 also comprises power measurement and faultdetector circuitry 338, memory 340, a Central Processing Unit (CPU) 342and a display 344. The form and function of the power measurement andfault detector circuitry 338, memory 340, Central Processing Unit (CPU)342 and display 344 are described above with reference to FIGS. 3 to 13.The apparatus 330 of FIG. 16 further comprises a Wide Area Network (WAN)connection 346 and a Home Area Network (HAN) connection 348. Thedescription provided above with reference to FIGS. 10 to 13 makesreference to communication with a remote location. Each of the WAN 346and the HAN 348 is an example of such communication. More specificallythe WAN 346 provides for communication with a utility, such as anelectricity supplier. The HAN 348 provides for communication with anetwork of known form and function which is installed in the home orbusiness premises and which is operative to provide for heating control,air conditioning control or the like. Where such a network comprises acontrol and display unit, the present invention is operative to make useof such an installed control and display unit, e.g. by displayingdetected fault conditions.

A second example of application of the present invention is shown inFIG. 17. The apparatus 360 of FIG. 17 is of a form appropriate forapplication of multiple power measurements with multiple fault detectionand interruption capabilities in a distribution box. The distributionbox may be a circuit breaker or fusebox in a residential home or adistribution box in a sub-metering environment, such as in apartmentcomplexes, commercial buildings and mobile home parks. The apparatus 360comprises plural current measurement apparatus 362 according to theinvention, with each current measurement apparatus 362 being operativeto make measurements, detect faults and provide circuit interruption fordifferent branches in the box. Each current measurement apparatus 362 isof a form and function as described above with reference to FIGS. 3 to13. The apparatus 360 also comprises a control module 364, whichcomprises a Central Processing Unit (CPU) 366, memory 368, a displayunit 370, local communications circuitry 372 and a Home Area Network(HAN) connection 374. Each current measurement apparatus 362 containslocal communications for sending status data, power consumption data andfault detect event data to the CPU 366 via the local communicationscircuitry 372. The local communications function is shared amongst allbranches and is operative according to a protocol that enables multipledevices to share the communications channel. The local communicationsarrangement comprises optical or another isolated communication linkswhere each current measurement apparatus is individually powered fromits respective mains supply to thereby maintain isolation. The HANconnection 374 provides for communication of data by way of a home areanetwork. The HAN connection 374 is operative to send and receive spacedapart data packets containing measurement data rather than sending andreceiving measurement data on an ongoing basis. Thus the home areanetwork is operated below bandwidth capacity to thereby reduce thelikelihood of network congestion and delays in effecting urgent networkdependent functions, such as operation of a circuit breaker following anarc or ground fault. The control module 364 is operative to collatepower consumption data from each of the plural current measurementapparatus 362 which is conveyed to the utility for reporting and billingpurposes. In addition the control module 364 is operative to provide forcommunication of fault and power consumption data to a home area networkby way of the HAN connection 374 for the purposes of display of faultconditions and for local power consumption metering. Furthermore thecontrol module 364 is operative to receive fault data from each of theplural current measurement apparatus 362 and to make a determination andrespond in dependence on such received fault data. More specifically thecontrol module 364 is operative to operate a circuit breaker by way ofthe HAN in at least one of the plural current measurement apparatus 362instead of the circuit breaker being operated locally by the currentmeasurement apparatus 362 itself. In addition the control module 364 isoperative to make a comparative analysis of the received fault data andto respond in dependence on the analysis. For example if all the currentmeasurement apparatus 362 report the same form of fault it may bedetermined that the faults have been caused by a lightning strike. Thecontrol module 364 may then operate the circuit breakers in the pluralcurrent measurement apparatus 362 in a predetermined order. In anotherapplication the control module 364 is operative in cooperation with aremote control centre to determine and to effect connection ordisconnection of the electricity supply by way of the circuit breakerscomprised in the plural current measurement apparatus. For example if aconsumer has failed to settle his electricity bill the control module364 is operative in dependence on control data received from the controlcentre to open a circuit breaker and thereby disconnect the consumer.According to another example if premises are now occupied after a voidperiod the control module 364 is operative in dependence on control datareceived from the control centre to close a circuit breaker and therebyconnect the new consumer. The local communication circuitry 372 isoperative to send back instructions and/or configuration data to atleast one of the current measurement apparatus 362. Fault and powerconsumption data is also displayed at the control module 364 on thedisplay unit 370. The apparatus 360 also comprises an ac to dc powersupply unit 376, which is operative to receive electrical power from themains electricity supply, to rectify the mains supply and to otherwiseprovide a regulated dc power supply to each of the plural currentmeasurement apparatus 362 and the control module 364.

A block diagram representation of current measurement apparatus 1000according to an eleventh embodiment is shown in FIG. 18. Components incommon with the embodiment of FIG. 10 are designated by like referencenumerals and the reader's attention is directed to the descriptionprovided above with reference to FIG. 10 for a description of suchcommon components. Components particular to the embodiment of FIG. 18will now be described. The current measurement apparatus 1000 comprisesa third shunt resistor 1004, a fourth shunt resistor 1002, a thirdacquisition circuit 1008, a fourth acquisition circuit 1006, a modifiedisolator 1010 and a common signal removal circuit 1012. The third shuntresistor 1004 is in series with the first shunt resistor 76 in the liveconductor 72 with the third shunt resistor 1004 being present in thelive conductor 72 on the load side of the series connected calibrationresistor 102 and switch 104. The fourth shunt resistor 1002 is in serieswith the second shunt resistor 78 in the neutral conductor 74 with thefourth shunt resistor 1002 being present in the neutral conductor 74 onthe load side of the series connected calibration resistor 102 andswitch 104. The third acquisition circuit 1008 is configured to acquirean analogue current measurement from the third shunt resistor 1004 andgenerate a digital representation of the analogue current measurement.The fourth acquisition circuit 1006 is configured to acquire an analoguecurrent measurement from the fourth shunt resistor 1002 and generate adigital representation of the analogue current measurement. The modifiedisolator 1010 receives outputs from the first and third acquisitioncircuits 82, 1008 to thereby provide for galvanic isolation between thelive and neutral circuits. The common signal removal receives an outputfrom the fourth acquisition circuit 1006, an isolated output from thethird acquisition circuit 1008, an isolated output from the firstacquisition circuit 82 and an output from the second acquisition circuit84. Otherwise and although not shown in FIG. 18 the embodiment of FIG.18 comprises the fault detection circuitry present in the embodiment ofFIG. 10 after the first and second processing circuits 90, 92.

Operation of the embodiment of FIG. 18 will now be described. Thecalibration source 80 in the form of the series connected calibrationresistor 102 and switch 104 is operative to apply a calibration signalto the live and neutral conductors which passes through the first andsecond shunt resistors 76, 78 but not the third and fourth shuntresistors 1004, 1002. The lack of calibration signal in the third andfourth shunt resistors 1004, 1002 provides a basis for removal ofundesired signals common to the calibration signal and signals presenton the live and neutral conductors. The common signal removal circuit1012 is operative to correlate the four input signals with one another.The common signal removal circuit 1012 is further operative to subtractthe signals from the first and third shunt resistors 76, 1004 from eachother and to subtract the signals from the second and fourth shuntresistors 78, 1002 from each other. One of the two differences is thensubtracted from the other difference to provide a factor relating to thecommon signal. The common signal removal circuit 1012 is then operativeto apply the common signal factor to each of the outputs from the firstand second shunt resistors 76, 78 to thereby remove the effects of thecommon signal. The thus corrected signals are then passed on to thecorrelation detection and correction circuit 88 and subsequent circuitryfor processing as described above. According to another approach, whichis of equivalent effect to the above described approach, the signalsfrom the first and second shunt resistors 76, 78 are subtracted fromeach other and the signals from the third and fourth shunt resistors1004, 1002 are subtracted from each other to provide two differenceswhich are then subtracted to yield a common signal factor. Consideringthe calibration signal further, the calibration signal on the liveconductor is removed by determining the average ofIcal(live)=Ilive(phase 2)−Ilive(phase 1) where Ical(live) is thecalibration signal present on the live conductor, Ilive(phase 2) is thelive current measured with the calibration signal present andIlive(phase 1) is the live current measured when no calibration signalis present. Similarly calibration signal on the neutral conductor may beremoved by determining the average of Ical(neutral)=Ineutral(phase2)−Ineutral(phase 1) where Ical(neutral) is the calibration signalpresent on the neutral conductor, Ineutral(phase 2) is the neutralcurrent measured with the calibration signal present andIneutral(phase 1) is the neutral current measured when no calibrationsignal is present. The error in a gain mismatch, A, between the live andneutral conductors is then determined. After application of the gainmismatch to all subsequent measurements and removal of the calibrationsignal the difference between the live and neutral current signals isdetermined.

A first form of current measurement arrangement 1100 is shown in FIG.19A. FIG. 19A shows an alternative configuration of calibration sourceand shunt resistors to the configuration of FIG. 10 or FIG. 18. Theconfiguration of FIG. 19A comprises the first and second shunt resistors76, 78 and a calibration resistor 1102. The configuration furthercomprises first, second, third and fourth switches 1104, 1106, 1110,1112 instead of the single switch of previous embodiments. The firstswitch 1104 connects a first end of the calibration resistor 1102 to theload side of the first shunt resistor 76 and the third switch 1110connects the first end of the calibration resistor 1102 to the sourceside of the first shunt resistor 76. The second switch 1106 connects thesecond opposite end of the calibration resistor 1102 to the load side ofthe second shunt resistor 78 and the fourth switch 1112 connects thesecond end of the calibration resistor 1102 to the source side of thesecond shunt resistor 78. A first pair consisting of the first andsecond switches 1104, 1106 are opened and closed together and a secondpair consisting of the third and fourth switches 1110, 1112 are openedand closed together and such that the first and second pairs of switchesare operated out of phase. The calibration current therefore eitherflows through or bypasses both of the first and second shunt resistors76, 78. Also at any one time there is always one pair of switches closedbetween the live and neutral conductors whereby the maximum voltage seenacross a switch is the voltage developed across a shunt resistor. Thisform of current measurement arrangement is used to remove signals commonto the calibration signal and the signals present on the live andneutral conductors as follows. The common signal removal circuit 1012 ofthe embodiment of FIG. 18 is operative to receive and correlate thesignals acquired from the two shunt resistors during both phases ofclocking of the switches. The common signal removal circuit 1012 is thenoperative to subtract acquired signals comprising the calibration signalfrom each other and to subtract acquired signals lacking the calibrationsignal from each other. Thereafter the common signal removal circuit1012 is operative to subtract one of the differences from the other tothereby determine a factor relating to the common signal which is thenused to remove the common signal from acquired signals comprising thecalibration signal.

A second form of current measurement arrangement 1120 is shown in FIG.19B. The configuration of the second form of current measurementarrangement 1120 is the same as the configuration of the first form ofcurrent measurement as shown in FIG. 19A. The second form of currentmeasurement arrangement 1120 differs from the first form of currentmeasurement arrangement 1100 in respect of how the four switches areclocked. In the second form 1120 a first pair consisting of the firstand fourth switches 1104, 1112 are opened and closed at the same timeand a second pair consisting of the second and third switches 1106, 1110are opened and closed at the same time and such that the first andsecond pairs of switches are operated out of phase. In common with thefirst form of current measurement arrangement 1100 the second form 1120there is always at any one time one pair of switches closed between thelive and neutral conductors whereby the maximum voltage seen across aswitch is the voltage developed across a shunt resistor. Currentmeasurement apparatus comprising the second form 1120 is operative toremove the common signal by performing two sets of subtractions onsignals acquired from the first and second shunt resistors as describedabove in relation to FIG. 19A. Therefore one obtains a first expression,Signal+Ical−A*Signal, during one phase and a second expression,Signal−A*(Signal+Ical), during the other phase where Signal is the loadcurrent signal, Ical is the calibration signal and A is the gainmismatch between the live and neutral conductors. A factor in A and Icalis obtained by determining the difference between the two expressionswhich is used as described above to provide for calibration.

A third form of current measurement arrangement 1140 is shown in FIG.19C. The configuration of the third form is the same as the first formof current measurement arrangement 1100 with the exception of thereplacement of the calibration resistor 1102 with a calibrationcapacitor 1142. The calibration capacitor 1142 is either an X or Y type.Otherwise the third form 1140 is operative with regards to the operationof the four switches either according to the first form of FIG. 19A orthe second form of FIG. 19B. The third form has the advantage over thefirst and second forms of dissipating substantially no active power.

A fourth form of current measurement arrangement 1160 is shown in FIG.19D. The fourth form comprises the first and second shunt resistors 76,78 and a calibration source consisting of a series arrangement ofmeasurement resistor 1164, calibration capacitor 1162 and a voltagesource 1170. The calibration source is connected between the live andneutral conductors on the load side of the first and second shuntresistors 76, 78. The fourth form further comprises a measurementconfiguration 1166 which is operative to measure a current signal in themeasurement resistor 1164. The measurement configuration 1166 thereforecomprises a sample and hold circuit and an analogue to digital converterwhich are operative to measure a voltage signal across the measurementresistor. In operation voltage source 1170 applies a sinusoidal voltageto the calibration capacitor 1162 and thereby causes a sinusoidalcalibration current to flow in the first and second shunt resistors. Thesinusoidal calibration current in the first and second shunt resistorsis measured and provides for calibration as described above. In otherforms voltage source 1170 is configured to apply a variety of waveformsother than sinusoidal waveforms. The measurement configuration 1166 isoperative to measure the calibration current flowing through themeasurement resistor 1164. Current measurement apparatus comprising thefourth form is operative to set a desired calibration current bycontrolling the voltage source 1170 in dependence on measurements madeby the measurement configuration 1166. Where the calibration impedanceis a resistor the amplitude of calibration signal varies with the linevoltage and so the SNR varies from measurement to measurement within acomplete cycle of the line voltage. In view of this the currentmeasurement apparatus is configured to weight measurements from a loadresistor in dependence on the line voltage signal. Where a reactivecalibration impedance such as a capacitor is used instead of a loadresistor the current measurement apparatus is configured to weightmeasurements differently within a cycle of the line voltage to takeaccount of the phase difference between voltage and current. Weightingof measurements with better SNR in preference to measurements withpoorer SNR within a cycle improves the overall SNR.

A third example of application 1200 of the present invention is shown inFIG. 20. The third example of application 1200 comprises a current andvoltage sensor block 1202, acquisition circuitry 1204, calibration andnormalisation circuitry 1206, a calibration source 1208 and faultdetection and power measurement circuitry 1210. The third example ofapplication 1200 also comprises a usage and fault state machine 1212, anarm/disarm command interface 1214, a switch and relay block 1216, a postswitch sensor block 1218 and a state display 1220. The current andvoltage sensor block 1202 comprises plural shunt resistors as describedin any previous embodiment and a line voltage sensor arrangement asdescribed above. The acquisition circuitry 1204 is operative to acquiresignals sensed by the current and voltage sensor block 1202. Thecalibration and normalisation circuitry 1206 is operative to provide forcalibration in dependence on application of a calibration signal by thecalibration source 1208 as described according to previous embodimentsand to normalise absolute and differential measurements. The faultdetection and power measurement circuitry 1210 is operative to determinepower consumption and to detect faults, such as ground and arcingfaults, in dependence on the normalised absolute and differentialmeasurements as described above. The switch and relay block 1216 isoperative to open and close one, other or both of the live and neutralconductors in dependence on operation of the fault detection and powermeasurement circuitry 1210. Operation of the switch and relay block 1216is under the control of the usage and fault state machine 1212 whichmakes decisions with regards to opening and closing the live and neutralconductors in dependence on operation of the fault detection and powermeasurement circuitry 1210 and on manual intervention by way of thearm/disarm command interface 1214. The arm/disarm command interface 1214may, for example, be used to test the apparatus in particular withregards to operation of the switch and relay block 1216. The apparatusis operative to provide a delay between power up and operation of theswitch and relay block 1216 to allow sufficient time for calibration andaccurate operation. The post switch sensor block 1218 is operative tomeasure current in the live and neutral conductors on the load side ofthe switch and relay block 1216. The output from the post switch sensorblock 1218 is provided to the usage and fault state machine 1212. Theapparatus is operative when the switch and relay block 1216 interruptsat least one of the live and neutral conductors to prevent reconnectionof the live and neutral conductors when at least one of the live andneutral conductor currents as sensed by the post switch sensor block1218 is unduly high. Reconnection is thereby prevented when, forexample, there is a short or mis-wiring on the load side. The statedisplay 1220 is operative to display by way of the like of an LCDdisplay the current status of the apparatus. The state display is alsooperative to provide for remote communication of the current status byway of a wired or wireless communications channel. The usage and faultstate machine 1212 comprises non-volatile memory which is operative tostore default data and to load the stored default data if power to theapparatus is interrupted. Alternatively default data is communicated tothe apparatus by way of the remote communications channel supported bythe state display 1220.

A block diagram representation of current measurement apparatus 1300according to a twelfth embodiment is shown in FIG. 21. Components incommon with the embodiment of FIG. 10 are designated by like referencenumerals and the reader's attention is directed to the descriptionprovided above with reference to FIG. 10 for a description of suchcommon components. Components particular to the embodiment of FIG. 21will now be described. The current measurement apparatus 1300 comprisesa current transformer 1302 (which constitutes a differential measurementdevice) around the live and neutral conductors on the load side of thecalibration source formed by the series connected calibration resistor102 and switch 104. The current measurement apparatus 1300 alsocomprises a current transformer data acquisition circuit 1304 which isconfigured to acquire and convert signals sensed by the currenttransformer 1302. The current measurement apparatus 1300 furthercomprises a modified correlation detection and correction circuit 1308which receives an input from the current transformer data acquisitioncircuit 1304 in addition to inputs from the first and second acquisitioncircuits 78, 82. In one form, a first end of the calibration source 102,104 is connected to the live conductor on the load side of the currenttransformer 1302 and a second opposite end of the calibration source isconnected to the neutral conductor between the second shunt resistor 78and the current transformer. The calibration source 102, 104 istherefore operative to apply a calibration signal which is sensed by thecurrent transformer allowing for calibration of the current transformeras well as the shunt resistors. In another form the first end of thecalibration source 102, 104 is instead connected to a length ofconductor which is passed through the current transformer 1302 from theload end and is then connected to the live conductor between the firstshunt resistor 76 and the current transformer. The length of conductorfrom the calibration source is operative to apply a calibration signalto the current transformer and the connection to the live wire betweenthe current transformer and the first shunt resistor provides for thepassage of the calibration signal through the first and second shuntresistors. In both forms the modified correlation detection andcorrection circuit 1308 is operative on the several acquired signals inthe same fashion as described above with reference to FIG. 10.

The current transformer 1302 provides for measurement of the sum of thelive and neutral currents and therefore provides an additional means ofmeasuring the difference in absolute measurements based on the first andsecond shunt resistors 76, 78. The combination of measurement approachesprovide for ease simultaneous power measurement and fault detection. Thecombination of measurement approaches also allows for ground fault andarc detection over different voltage ranges and difference frequencyranges. Furthermore combination of measurement approaches provides forenhancements to ground fault detection. Also such current measurementapparatus provides for ease of provision of the like of AFCI and groundfault detection functions by relying more on measurements made by thedifferential measurement device in preference to absolute measurementswhen the currents on live and neutral are high and relying more onabsolute measurements in preference to measurements made by thedifferential measurement device when the current difference between thelive and neutral conductors is great.

A block diagram representation of current measurement apparatus 1320according to a thirteenth embodiment is shown in FIG. 22. Components incommon with the embodiment of FIG. 21 are designated by like referencenumerals and the reader's attention is directed to the descriptionprovided above with reference to FIG. 21 for a description of suchcommon components. Components particular to the embodiment of FIG. 22will now be described. The current measurement apparatus 1320 lacks thefirst shunt resistor 76 of the embodiment of FIG. 21 and the first shuntresistor's processing chain. The current measurement apparatus 1320comprises only the second shunt resistor 78 in the neutral conductor. Inaddition the current measurement apparatus 1320 comprises areconstruction, correlation and calculation circuit 1322 instead of themodified correlation detection and correction circuit 1308 of theembodiment of FIG. 21. The reconstruction, correlation and calculationcircuit 1302 is operative to correlate signals acquired from the secondshunt resistor 78 and the current transformer 1302 and to determine thelive current signal by subtracting the acquired neutral current signalfrom the differential signal acquired by the current transformer. Thereconstruction, correlation and calculation circuit 1302 then passes theacquired neutral current signal, the determined live current signal andthe acquired differential signal onto the remaining processing circuitrywhere power consumption is determined and fault conditions detected asdescribed above with reference to previous embodiments.

The invention claimed is:
 1. Current measurement apparatus comprising: afirst current sensor for sensing a current in a first conductor; a firstmeasurement circuit configured to buffer an output of the first currentsensor to provide a first buffered signal having a first dynamic rangeand to measure the first buffered signal to provide a first measurementsignal; and a second measurement circuit configured to buffer the sameoutput of the first current sensor to provide a second buffered signalhaving a second dynamic range different from the first dynamic range andto measure the second buffered signal to provide a second measurementsignal.
 2. The current measurement apparatus of claim 1, wherein thefirst measurement circuit includes a first gain stage and a first analogto digital converter and the second measurement circuit includes asecond gain stage and a second analog to digital converter; and whereina signal range of the first analog to digital converter is larger than asignal range of the second analog to digital converter.
 3. The currentmeasurement apparatus of claim 2, wherein the first measurement circuitis configured for use in arc fault detection in a power distributionnetwork and the second measurement circuit is configured for use in atleast one of ground fault detection and metering of current flow.
 4. Thecurrent measurement apparatus of claim 2, wherein the first and secondmeasurement circuits are configured for clocking at differentfrequencies.
 5. The current measurement apparatus of claim 1 furtherincluding: a second current sensor for sensing a current in a secondconductor and a third measurement circuit responsive to an output of thesecond current sensor; where the first and second conductors act as thesupply and return wires to a load, such that under no fault conditionsthe currents in the first and second conductors are the same.
 6. Thecurrent measurement apparatus of claim 5 further including: circuitryoperative to perform calibration in respect of the current pathsassociated with the first and second current sensors to align ameasurement signal produced by the third measurement circuit and ameasurement signal produced by at least one of the first and secondmeasurement circuits.
 7. The current measurement apparatus of claim 6further including: circuitry to introduce a calibration signal to causea shared perturbation in currents measured by the first and secondcurrent sensors, such that transfer functions associated with the firstand second current sensors can be adjusted to have substantiallymatching responses to the calibration signal or such that correctivefactor can be applied.
 8. The current measurement apparatus of claim 7further including: a correlation and detection circuit that is operativeduring calibration to extracts parts of a first signal from one of thefirst and second measurement circuits, the first signal containing acontribution from the calibration signal and a contribution from acurrent passing along the first conductor; and the correlation anddetection circuit being further arranged to extract parts of the asecond signal from the third measurement circuit, the second signalcontaining a contribution from the calibration signal and a contributionfrom the current passing along the second conductor, the extracted partsrelating to the calibration signal being used to adjust the transferfunctions; and at least one processing circuit operative to determine amagnitude of a current flowing in the first conductor and a magnitude ofa current flowing in the second conductor.
 9. The current measurementapparatus of claim 8, wherein the processing circuit is configured tovary its determination of a fault condition based on a magnitude of thecurrent being drawn by the load.
 10. The current measurement apparatusof claim 5, wherein one of the first and second current sensors is adifferential current measurement device configured to provide a sum ofthe of the current in the first and second conductors.
 11. The currentmeasurement apparatus of claim 1 further including: a voltagemeasurement circuit to measure the voltage of the first conductor withrespect to a reference voltage.
 12. The current measurement apparatus ofclaim 3, wherein the first conductor is a live conductor carrying afirst phase of a power distribution network, the apparatus furthercomprising a voltage measurement circuit for measuring a voltagedifference between the first conductor and a second conductor; andwherein the second conductor is one of a second phase and a neutral ofthe power distribution network, and wherein a voltage signalrepresentative of the voltage between the first and second conductors isanalyzed for arc fault detection.
 13. The current measurement apparatusof claim 3 further including: a second current sensor for sensing acurrent in a second conductor, a third current sensor for sensing acurrent in a third conductor and a fourth current sensor for sensing acurrent is a fourth conductor, where the first to third live conductorsand a neutral conductor are conductors of a three phase supply.
 14. Anapparatus for ground fault detection and/or power metering, comprising:a first current transducer responsive to a difference in magnitudebetween currents flowing in live and neutral conductors connecting adevice to a supply, wherein the device comprises a load or power source;a second current transducer configured to measure the magnitude of thecurrent in the neutral conductor; and a circuit configured to measurepower consumption and/or to detect a fault condition based on firstinformation from the first current transducer about the difference inmagnitude between currents flowing in the live and neutral conductorsand second information from the second current transducer about themagnitude of the current in the neutral conductor.
 15. The apparatus forground fault detection and/or power metering of claim 14 furtherincluding: a second circuit arranged to correlate signals from the firstand second current transducers and to determine information about acurrent in the live conductor by subtracting the acquired neutralcurrent acquired by the second transducer from the differential signalacquired by the first current transducer.
 16. The apparatus for groundfault detection and/or power metering of claim 14 further including: athird current transducer arranged to measure the magnitude of thecurrent flowing in the live conductor, wherein estimates of differencesbetween the currents in live and neutral conductors are provided byforming a difference between the magnitudes of current as determined bythe second and third current transducers and also from output of thefirst current transducer thereby allowing the transducers to be selectedto enhance fault detection over different current and frequency ranges.17. The apparatus for ground fault detection and/or power metering ofclaim 14 further including: a calibration circuit for introducing acalibration signal which causes a shared perturbation in the currentsmeasured by the first and second current transducers, such that transferfunctions can be adjusted to have substantially matching responses tothe calibration signal or such that corrective functions can be applied.18. The apparatus for ground fault detection and/or power metering ofclaim 17 further including: a correlation and detection circuit that isoperative during calibration to extracts parts of a first signal fromone of first and second measurement circuits, the first signalcontaining a contribution from the calibration signal and a contributionfrom a current passing along the live conductor; the correlation anddetection circuit being further arranged to extract parts of a secondsignal from a third measurement circuit, the second signal containing acontribution from the calibration signal and from the current passingalong the neutral conductor, the extracted parts relating to thecalibration signal being used to adjust the transfer functions, and atleast one processing circuit operative to determine a magnitude of acurrent flowing in the live conductor and a magnitude of a currentflowing in the neutral conductor.
 19. The apparatus for ground faultdetection and/or power metering of claim 14, wherein the circuit isconfigured to determine information about a current in the liveconductor by subtracting a magnitude of the neutral current as measuredby the second current transducer from a magnitude of a differentialsignal as measured by the first current transducer.
 20. A currentmeasurement apparatus comprising: a first current sensor for sensing acurrent in a first conductor; a first measurement circuit responsive toan output of the first current sensor and having a first dynamic range;and a second measurement circuit responsive to the output of the firstcurrent sensor and having a second dynamic range different from thefirst dynamic range; wherein the first measurement circuit includes afirst gain stage and a first analog to digital converter and the secondmeasurement circuit includes a second gain stage and a second analog todigital converter; and wherein a signal range of the first analog todigital converter is larger than a signal range of the second analog todigital converter.
 21. The current measurement apparatus of claim 20,wherein the first measurement circuit and the second measurement circuitare clocked at different frequencies.
 22. The current measurementapparatus of claim 20, wherein the first measurement circuit isconfigured for arc fault detection.
 23. The current measurementapparatus of claim 20, wherein the second measurement circuit isconfigured for ground fault detection or for metering current flow.